TEXAS AGRICULTURAL EXPERIMENT STATION R. D. LEWIS, Director. College Station. Texas Bul/elin 73o Fertiiity Relationships in Maize-Teosinte Hybrids JOHN S. ROGERS Department of Agronomy LIBRARY n. a M. couzee 0F TEXAS G/vwanéw 1950 ‘A. if; The TEXAS AGRICULTURAL AND MECHANICAL COLLEGE SYSTEM j GIBB GILCHRIST. Chancellor Digest This bulletin reports the degree of fertility found in crosses maize with several different varieties of teosinte. The teosint: used were the Mexican varieties Durango, Chalco, Nobogaml and New, and the Guatemalan varieties Huixta and Florida. i Hybrids of the Mexican teosinte varieties with maize exhib approximately normal fertility, although the percentage fertili ' increases slightly with the use of the Chalco, Nobogame m Durango varieties, respectively. Hybrids of maize with the Hu' _ variety of northern Guatemala exhibit relatively normal fertili ~ 1 although the percentage is somewhat below that found in if Mexican teosinte-maize hybrids. In contrast. the F1 hybrid» maize with the Florida variety of southern Guatemala shows -g A average pollen fertility of 49 percent. v A study of several backcross populations involving the Flori teosinte variety reveals that the maize parent has a pronounc effect on the degree of sterility, and that a definite linka exists between sterility and marker genes on chromosome ; These results indicate that some factor or factors on the fo chromosome, located very close to Tu, and between Tu and =' account for most of the sterility in this hybrid involving Florida variety. Results from this experiment, as well as the findings of o workers, indicate that the Mexican teosintes, northern malan teosintes and maize are similar in chromosome struct = In addition, the existing evidence indicates that differences the fourth chromosome are of primary importance in distin ing the southern Guatemalan teosintes from maize, and to -'"{ extent from the other teosinte varieties. "t BULLETIN 730 NOVEMBER 1950 Fertility Relationships in Maize-Teosinte Hybrids IOHN S. ROGERS Associate Professor. Department oi Agronomy MAIZE IS A MEMBER of the grass family Gramineae, and all types are classified within a single species, Z ea mag/s L. Annual teosinte, which is the closest known relative of maize, has the same chromosome number, 10, and produces fertile hybrids when crossed with maize. Teosinte was originally assigned the generic rank of Euchlaena Schrad, as separate from Zea L., but Reeves and Mangelsdorf (11) have proposed a revision making EuehZae/na and Zea congeneric, and suggest- ed that annual teosinte be designated as Zea mexicana (Schrad.) R. & M. Fertile hybrids between maize and teosinte were first produced by Harshberger (3), who called attention to this as proof that they were close relatives. Studies of maize-teosinte hybrids have been conducted by numerous investigators since that time, and all have shown that the two species may be crossed easily and fertile hybrids obtained. Although the maize-teosinte hybrids so far investigated have proved fertile, the degree of fertility has been found to depend somewhat on the variety of teosinte used in the cross. Likewise, cytological observations of hybrids involving certain varieties of teosinte have shown that the chromosomes do not always pair normally. Kuwada (4) reported that the F1 hybrids of maize and Florida teosinte, a commercial variety which was originally obtained from southern Guatemala, had regularly 10 pairs of chromosomes, but in two of the pairs the chromosomes were of different lengths. Longley (5), in a study of the F1 hybrids of maize and the Chalco variety from Mexico, found 10 bival- ents of regular behavior at meiosis. Beadle (2) studied F1 hybrids of several varieties of teosinte with maize, and found i " no irregularities during meiosis in crosses involving the Mexi- can varieties Durango and Chalco. However, in Florida teo- sinte-maize hybrids two unpaired bivalents were frequently present, and sometimes as many as four were found. Pollen sterility in this hybrid was approximately 32 percent. In a 4 BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION cross between Durango and Florida teosinte the pollen sterili»; averaged as high as 66.8 percent, and examinations at meio showed more irregular behavior than was found in the Flori teosinte-maize hybrid. Mangelsdorf and Reeves (9) studi, the fertility in hybrids of Florida and Durango teosinte wi maize, and reported that the Florida teosinte-maize hybri produced approximately 50 percent sterile pollen, while f Durango teosinte-maize hybrids exhibited normal fertility. j Detailed cytological investigations have produced eviden. i as to the exact chromosomes involved in the irregular behavi. of certain maize-teosinte hybrids. In a study of pairing rel tions between corn and teosinte chromosomes, through the L, of translocations which served as chromosome markers, A l {f son (1) found that chromosomes 1, 2 and 6 behaved regula in both Durango and Florida teosinte hybrids with maize, v did also chromosome 7 in a Florida teosinte-maize hybr'i Two heteromorphic pairs were observed in the cross Wi Florida teosinte, but they were not identified, althou chromosome 5 was suspected of being one of the uneq pairs, and chromosome 8, 9 or 10 the other. Longley (6) studied the chromosome morphology of se eral varieties of teosinte from Mexico and Guatemala. reported that chromosomes of the Mexican teosintes m’ nearly resembled those of maize, while chromosomes of t teosintes from northern Guatemala were somewhat t, maize-like than those from southern Guatemala. A cytologi,’ examination of chromosomes in Florida teosinte-maize hybri revealed that chromosomes 5 and 9 of teosinte were consi ently longer than their maize homologs, and that the lo arm of these two chromosomes did not always pair complete Longley (7) also reported that from one to four chromoso bridges are not uncommon in Florida teosinte-maize hybri indicating that there may be several differences in the lin” arrangement of the genes between the two parents. ‘ O’Mara (10) studied hybrids of maize with the Gua malan varieties Nojoya, Moyuta and Florida, and found t v the Nojoya hybrid was quite normal at meiosis, but that f latter two were somewhat irregular. Two and four univale were found in both the Florida and Moyuta hybrids w’ maize, and bridges or fragments were often present at me phase. Cytological observation of a Florida-Nojoya teosi" hybrid revealed the same type of behavior as found in t Florida teosinte-maize hybrid. In addition, a detailed analy of each chromosome in this hybrid indicated, that with res i to maize, Florida teosinte had an undetermined rearrangem‘; FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS 5 in the long arm of chromosome 4, as well as rearrangements Eon chromosomes 8 and 9. The Nojoya teosinte-maize hybrid iproduced normal pollen, while approximately one-third of the pollen was deficient in starch in the hybrid of Florida teosinte l-with maize. .4 These previous studies of maize-teosinte hybrids reveal that the teosinte varieties of Mexico and northern Guatemala ijiroduce hybrids with maize of normal fertility and regular meiotic behavior. The varieties from southern Guatemala, Ehowever, when crossed with maize, have in all instances shown marked degree of sterility in addition to irregular meiotic lbehavior. 4 The experiment reported in this bulletin was conducted o determine the degree of sterility in F1 hybrids between aize and several teosinte varieties, and if possible to deter- p ine the mechanism and chromosomes leading to this partial “terility in certain maize-teosinte hybrids. Materials and Methods __ The teosintes used in this study were the Mexican varie- f=ies Durango, Chalco, Nobogame and New, and Huixta from orthern Guatemala, and Florida, an importation to this untry from southern Guatemala. In addition to the F1 ybrids of these teosintes and maize, several maize backcross pulations, as well as several inter-varietal teosinte hybrids, ere studied. , Since it has been shown that Florida teosinte-maize ybrids exhibit a considerable degree of sterility, a more ,_ ensive study was made with this particular variety of gsinte. Maize stocks with marker genes were crossed with lorida teosinte, and the F1 hybrids were backcrossed to cessive maize stocks. In this manner, by classification of p, e backcross progenies, any linkage of pollen sterility with arker genes on particular chromosomes might be determined. v ll chromosomes, with the exception of the second and seventh, fere tested by the use of such genes. Samples from all plants studied were obtained by shaking ‘llen into small glassine bags so that it might be stored and ' amined at- a later date. Examination of the pollen was ade by mounting the samples in lactic acid to which a few ‘drops of iodine had been added. This procedure restored the ains to their normal size, and made the normal type dis- hguishable from those which had aborted. Slides were 6 BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION made of the pollen collected from each plant in all populations, and approximately 300 grains were counted on each slide.- All grains staining dark and appearing to be of normal siz were counted as fertile, and all other types were designate as sterile. In this latter class three main types could be di, tinguished: those completely devoid of starch and often colf lapsed, grains of approximately normal size but with only small amount of starch, and grains below normal in size an usually somewhat deficient in starch content. After coun p were made from samples of all plants, the percentage fertilit was determined for each individual plant. i Mexican Teosinte-Maize Hybrids ‘Z Pollen examined from the F1 hybrid of Durango teosin x maize was normal in appearance. An average of 99; percent fertile pollen was obtained in a count of 25 plani Pollen was also examined from a large number of plants ' two (Durango teosinte x maize) x maize backcross popul’ tions, and none of the plants exhibited an abnormal degree y" sterility. About three-fourths of the plants examined in bo progenies were above 95 percent in fertility, while the othe were only slightly below this figure. i‘ The F1 hybrids of Chalco teosinte x maize also exhibi " relatively normal fertility, although the average percent i. fertile pollen was ‘slightly less than that for the Dura teosinte-maize hybrids. Samples from 30 plants gave l‘ average of 95.8 percent fertile pollen. 1 These results indicate that the chromosomes of the Ch ' variety, although still quite similar to those of maize, di to a slightly greater degree than do those of the Dura variety. Certain gene combinations may occasionally be A tained through segregation which do not permit normal p0 development. It is also possible that irregularities in mei may sometimes occur due to small differences in homol‘ between the parental chromosomes. At least it seems re able to conclude that the chromosomes of the two pared’ types do not always complement each other, and that s‘ differences lead to the slightly below-normal fertility f0 in the Chalco teosinte-maize hybrids. A somewhat different situation from that found in ei the Durango or Chalco teosinte-maize crosses was encoun _ on examination of the Nobogame teosinte-maize hybrid. A the 30 F1 plants examined, 24 were essentially normal wit: FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS Pollen from plants of the F1 hybrid Nobogame teosinte x maize. A. Normal pollen. B. Semi-sterile pollen. Figure 1. average fertility of 96.7 percent, while the other 6 were semi- p rile with an average fertility of 50.2 percent. The abortive fllen of the semi-sterile plants was usually small with very le starch, although a few of the grains were completely pty. Pollen samples from both types of plants are shown Figure 1. It was evident from this examination theatythe obogame teosinte used as a parent was segregating for some Actor causing small or empty pollen. A reciprocal trans- tion would behave in this manner; however, no cytological idence has been obtained on these semi-sterile plantsiwhich ight verify this possibility. The average pollen fertility in e normal F1 plants of this cross, although higher than? that the maize-Chalco hybrid, is still lower thanzthe fertility the maize-Durango hybrid. Apparently the Nobogame ‘riety is somewhat intermediate between the Durango and t lco varieties on the basis of their chromosomal homology pith respect to maize. p. In addition to the F1 hybrid, two backcross progenies of obogame teosinte x maize) x maize were examined for llen sterility. All plants of the first progeny examined were rmal in pollen fertility. Evidently a normal F1 plant had ien used in making this backcross to the maize parent. The her Nobogame teosinte-maize backcross examined, however, mowed a segregation for normal and semi-sterile plants. Of d 50 plants examined, 20 were of the normal type and 3O are semi-steriles. The former group averaged 96.2 percent irtile pollen, while the latter averaged 59.4 percent. This ihavior indicates that a semi-sterile plant was used in mak- this backcross, and that the semi-sterile condition is ‘herited as a dominant character. The ratio of normal to mi-sterile plants does not depart significantly from a 1:1 itio, so it is entirely. possible that one factor, such as a reci- ocal translocation, is responsible for this condition. I 8 BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION The results obtained in this study with the Mexica teosintes corroborate the findings of previous investigato a All Mexican varieties so far studied, when crossed with maiz produce F1 hybrids with a high degree of fertility. Th' indicates that the chromosomes of these teosintes and mai i are quite similar, and that normal pairing ordinarily occu ‘ at meiosis. This latter fact has been established by all cyt logical studies made on hybrids including any of the Mexicali teosintes and maize. The semi-sterile condition noted in th' experiment in certain plants of the Nobogame teosinte-mai v F1 hybrid is apparently an exception to the general condition, and resulted from the use of a particular plant which was not representative of the variety. Guatemalan Teosinte-Maize Hybrids Samples from 17 plants of a Huixta teosinte-maize hybri, revealed an average pollen fertility of 89.7 percent. Thi‘ percentage is definitely below that ordinarily found in eithe’ corn or teosinte, and indicates there is sufficient differen _ between the parental chromosomes to cause some sterility Although no cytological studies of Huixta teosinte-mai f. hybrids have been conducted, O’Mara (10) reported consta i? and regular pairing in a hybrid of maize with a variety fro Nojoya in northern Guatemala which is quite similar , Huixta. This indicates the sterility of the maize-Huixt a hybrid does not occur asea result of irregularities at meios’ but is caused by gene differences between the two paren #- forms. It-is possible that segregation in this F, hybrid resul ‘ in a few disharmonious gene combinations which bring abo o abnormal pollen development and consequent sterility. Ce o, tainly, this lower fertility in the Huixta teosinte-maize hyb a; indicates that chromosomes of the Huixta variety differ fro maize chromosomes to a greater extent than do those of t Mexican varieties studied. The Florida teosinte-maize hybrid was the only one a5} the crosses studied to exhibit a consistently high degree u; sterility. The average pollen fertility for the 15 plants exa pf ined was 49 percent, and fertility among individual plan varied from 37.0 to 60.0 percent. Pollen samples from bo parents and the F1 hybrid are shown in Figure 2. The nine backcross populations involving Florida teosin show that an extreme amount of variability in pollen sterili ’ exists among the various crosses. The classification of a progenies, according to the percentage of plants in each popui’ lation which occurs at different fertility levels, is shown .% FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS 9 Figure 2. Pollen from plants of A. maize, B. Florida teosinte and C. Florida teosinte x maize. Table 1. In some progenies, more than half the population is above the 90 percent level of fertility, While practically no plants aproach the 50 percent level. On the other extreme, one of the crosses has practically no plants above 90 percent fertility, while almost half of the population is either ap- proaching or below the 50 percent level. Other progenies are intermediate between these types. s These observations indicate clearly that the maize stocks have a decided effect upon the amount of sterility inthe backcross population. These populations show that‘ in most cases there is a very small number of plants with as great an amount of sterility as is found in the F1 hybrids, which indicates that more than one factor, as well as an interaction of factors, may be the cause of this sterility produced between hybrids of the two species. Because of the continuous dis- tribution, as shown in Table 1, it has been impossible to divide Table 1. The percentage of plants. in each (Florida teosinte x maize) x maize progeny at different pollen fertility levels , Total plants 103 74 54 131 68 77 . 335 162 786 Percent < pollen Percentage of plants fertility 95-100 58.9 5.4 . 90-95 24.3 12.2 - 05-90 11.7 21.0 HooNn-l .P‘<°:‘>E\°S=E*° OOOOr-lcocnoodim e-Hwco oooywwwpwpg N H O5 HHOQN: EOCDEQEQOQ-‘QDOOOM 80-85 10.7 14.9 75-80 6 70-75 2 65-70 1 60-65 1. 0 1 0 l-l ¢ m www @“w@P@° mohbwhma F-‘l-ildl-‘IYJ @?°@9?FP@P@ ww wwqqawaw wwmww ...P?PP?FWP mwmmmwwommm wwmmwhhh 55-60 50-55 Below 5O m I I mwsmqwoowm Wco. w m w segregating had been grown. is l0 BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION these backcross progenies into exact classes according to the degree of fertility, as most 0f the plants are clustered in the upper range of fertility without an apparent dividing line. It was the original intention to classify these backcroi progenies into certain fertility groups, so that linkage values might be obtained between pollen sterility and the variou marker genes. However, as no natural division presented itself in the various populations, it became evident that a' arbitrary division would be necessary. All plants in eachf population were first arranged in descending order according to their percent fertility. Each population was then divided! into five groups of equal size, the first group containing the’ plants with the highest fertility, the second group those with the next highest fertility, and so on. By making these divij sions it was possible to determine Whether any linkages exist between sterility and the marker genes; for if such a linkag existed, the teosinte allele of the marker gene exhibiting th linkage would be expected to appear less frequently in th“ upper fertility groups and more frequently ‘in the lower fer tility groups. If no linkage existed, the marker genes woul_ be expected to occur with approximately "the same frequenc at each of the five fertility levels. _ _ ;~. The results of the division of these different progeni, are shown in Table 2. Tests for linkage werelmade by call’ culating chi-square values on the basis of independent asso I ment of the marker genes within the five fertility levels. The P values obtained by this method for each of the progeni studied are also included in Table 2. ' The gene Tu on chromosome 4 is the only marker gen studied which exhibits a highly significant deviation fro, the values expected by independent assortment. This devi; tion is in the direction expected if a linkage exists betwe 1 this gene and sterility. As there is an indication of linka‘ with the sul and 9Z3 genes, also on chromosome 4, it see W certain that this chromosome is involved with some sterilit“ factor. There is a definite tendency forthe plants carryi the teosinte alleles of these two genes to appear in the low fertility levels more often than would be expected by chanc and significant deviations would in all probability have be, obtained if larger populations in which these genes We No marker genes on the other chromosomes show devi tions which are significantly different from those expect by random assortment. The gene lg2 exhibits a deviatio‘ Table 2. FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS Linkage relations 0f marker genes with pollen sterility 11 Number of plants in each of Marker five fertility levels P Chrom. gene 1 2 3 4 5 Total Value 1 P 50 50 53 61 48 262 p 50 49 46 38 51 234 496 .37 1 bma 8 7 6 5 5 31 Bm; 3 4 5 6 5 23 54 .70 3 a1 14 14 16 12 13 69 A, 12 12 10 14 12 6O 129 .90 3 lg; 11 5 15 14 12 57 Lga 15 21 11 12 13 _ _ . I ' 129 .05 4 su, 7 7 4 2 2 22 5g Sui 7 7 10 11 11 I 46 .. H. 68 .11 4 g1. 7 5 2 2 3-3 19 G13 7 9 12 11 10 L 49 “ ' ‘ _ 68 .19 _._ 4 Tu 6 6 0 1 or!" .13 tu 12 11 17 16 17 73 86 .01 5 bml 8 8 7 8 7 38 Bml 8 8 8 7 8 39 .1 77 .99 6 Y 17 26 14 15 18 90 y 24 15 26 25 21 111 201 .08 6 Pl 74 76 64 60 58 332 pl 52 50 61 65 66 294 626 .10 8 jf 1 2 1 4 1 9 J. 10 9 10 7 9 45 54 .36 9 wx 3 2 5 4 2 16 Wx 8 9 6 7 8 38 54 .60 10' g1 5 6 3 3 6 23 G1 6 5 8 8 - 4 _- 31 54 .41 ‘Segregating in 3:1 ratio. 12 BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION which approaches significance, but its distribution in the various fertility levels does not show the definite trend that 5 might be expected if a linkage existed, and the deviation which does occur is in the wrong direction to indicate linkage. There is a slight indication of linkage between sterility and the i genes Y and Pl on chromosome 6. However, as the popula- = tions in both crosses are quite large, and the distribution of _ the marker genes in the five fertility levels does not show a 1 strong trend suggesting linkage, if a linkage does exist it is 1 certainly very slight. Another striking feature concerning the genes on chromo- a some 4 is the marked deficiency of the maize alleles exhibited f by the backcross populations. Only 15.1 percent tunicate, 1 30.7 percent glossy and 33.3 percent sugary segregates were recovered. Similar results were obtained by Mangelsdorf and ff Reeves (9) when they recovered 15.1 percent tunicate and *- 38.7 percent sugary segregates in the same type of backcross f populations. Although the cause of these deficiencies cannot be determined from these data, it seems reasonable to conclude a that they are related to the sterility which is found in these gl maize-teosinte hybrids. Certainly, the discovery of a linkage a between sterility and these genes on chromosome 4 lends credence to this supposition. Inter-Varietal Teosinte Hybrids In addition to the pollen sterility studies of various maize- teosinte hybrids, a study was made of F1 hybrids between i several varieties of teosinte. The varieties used in this study if were New, Florida, Durango, Nobogame and Chalco. The i last three are Mexican teosintes and the Florida variety is of Guatemalan origin. The native habitat of the New teosinte is f unknown, as it was originally obtained from the U. S. Depart- ment of Agriculture without any data on this point. In gross V‘ morphology and appearance, however, it resembles a great deal the Mexican teosinte varieties. It was originally intended to make all possible crosses l’ between these varieties, but due to differences in blooming dates it was possible to make only a part of them. Relatively few seed were obtained from each cross, and as all of these j did not germinate successfully, only a few hybrid plants were " obtained. All were examined for pollen fertility, and the c‘ results are given in Table 3. The pollen sterility in these hybrids suggests that the ‘l; Nobogame, Durango and New varieties are very similar in FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS 13 Table 3. Percent pollen fertility for all plants of inter-varietal teosinte hybrids Percent fertility of individual plants Nobogame Nobogame Durango New Nobogame Florida Florida New Durango New Chalco Chalco Chalco New 97.0 93.9 96.0 95.3 94.7 83.8 61.5 96.7 93.8 95.2 95.1 94.1 79.5 55.5 95 9 93.3 93.8 92.8 91.2 66.7 50.8 95.7 93.1 92.9 90.8 89.9 55.0 50.7 95.6 92.5 49.2 85.4 88.6 48.9 48.5 95.5 92.1 82.4 85.1 39.0 46.6 95.0 91.1 78.0 81.7 37.5 31.4 94.2 87.7 62.3 79.3 28.9 28.9 92.7 72.2 20.0 92.4 68.9 89.9 85.4 Ave. 93.8 92.2 85.4 85.3 84.6 51.0 46.7 chromosome structure. All hybrid plants involving only these three strains, with one exception in the New x Durango cross, show a rather high degree of fertility. The Chalco variety seems to be somewhat different from these three varieties, as in hybrids of Chalco with either New or Nobo- game teosinte, the pollen fertility is inclined to be lower than in crosses where the others alone are involved. Those hybrids with the Florida variety exhibit the greatest amount of steril- ity; hybrids of both the Chalco and New varieties with Florida show an average pollen fertility of approximately 50 percent. It should be noted, however, that there is a great range in fertility among plants of these last two crosses, indicating that the degree of sterility must be controlled by some easily changeable mechanism. This study indicates that the Mexican teosinte varieties, as well as New teosinte, are similar in chromosomal consti- tution, although thelChalco variety differs slightly from the other three. This finding is also in agreement with the re- sults from the maize-teosinte hybrids, as in these crosses the Chalco variety apparently differed from maize slightly more than either the Nobogame or Durango varieties. Both Chalco and New teosinte differ in some manner from Florida teosinte, as is shown by the high degree of sterility in hybrids of these two with the Florida variety. Durango is also known to differ from the Florida variety, as Beadle (2) reports an average pollen sterility of 66.8 percent, as well as irregular behavior at meiosis, in a hybrid of Durango x Florida teo- sinte. As Nobogame teosinte is known to be similar to the three‘ varieties which differ in some manner from Florida variation in type from the remainder 0f the species a l4 BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION teosinte, it seems certain that the Nobogame variety a differs from the Florida variety, although no hybrids betw, the two have actually been studied. It may be conclud. therefore, since the Mexican teosintes produce partially ste I, hybrids when crossed with Florida teosinte, that in gr‘ chromosomal morphology they resemble maize more clos than they do Florida teosinte. Discussion The pollen sterility studies conducted in this experim, corroborate previous findings on fertility relationships if maize-teosinte hybrids. Apparently all Mexican varieties g similar to maize in chromosomal structure, because their i brids with maize show regular behavior at meiosis and; normal degree of fertility. Although one exception was no in this study, when a high degree of sterility appeared? certain hybrids involving Nobogame teosinte, this behav is not characteristic of the Mexican varieties. It is also evid that the Mexican varieties, as Well as maize, differ from l southern Guatemalan varieties in chromosome structure, _. cytological investigations of Longley (6, 7) and O’Mara (i in addition to the pollen sterility studies, show that aildiff‘ ence in chromosomal homology exists between the teosintes i northern Guatemala and those of southern Guatemala. l; may be concluded, therefore, that teosinte varieties of sou ern Guatemala possess a particular chromosome structf which differentiates them not only from maize, but from other teosintevarieties which so far have been studied. ’ Although the various teosinte varieties differ somewi, in gross plant morphology, they are all quite similar in ‘ characteristic features which distinguish them from mal However, within this morphologically similar group of Va ties, there exists a difference in chromosome structure W is greater than that "found between most of the teosinte va ties and maize. In other words, if fertility of thexFl hyb is used as a basis of relationship, many of the teosinte varie might be considered more closely related to maize t l to certain other teosinte varieties. A comparison of inflo v cence characters, however, leaves no doubt but that all knot forms of teosinte may be placed in the same species, and t; the species should probably, as suggested by Reeves f Mangelsdorf (11), be made congeneric with maize. ‘Q group of teosintes from southern Guatemala represents/l perhaps, differs in chromosome structure as a result FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS 15 isolation from maize and other teosinte varieties for a long time. The exact nature of the chromosome structure of the southern Guatemalan teosinte varieties, as well as the cause for sterility in hybrids involving this group of varieties, is not yet fully understood. In any attempt to arrive at an explanation of the distinctive behavior of these particular varieties, however, it is essential to consider all pertinent facts now available on hybrids involving representative va- rieties of this group. In all instances, these hybrids show irregular chromosome behavior at meiosis, and two or four univalents are frequently present. Pollen sterility in these hybrids is always quite pronounced, and usually approaches 50 percent. In all backcross populations involving Florida teosinte, in which marker genes on chromosome 4 are segregating, there is a marked deficiency of the maize alleles. A definite linkage has been established between sterility and the marker genes on the fourth chromosome. It is of especial interest that the Tu gene shows not only the greatest deficiency of any of the fourth chromosome genes so far studied, but also the strongest linkage with sterility. Mangelsdorf and Reeves (9) and Mangelsdorf (8) showed conclusively that chromo- some 4 carries many of the genes responsible for the morpho- logical differences observed between maize and teosinte. In addition, O’Mara (10) reported that in respect to maize, there is a rearrangement on chromosome 4 of the southern Guate- malan varieties. Certainly, there is now a considerable b-ody of evidence indicating that differences on the fourth chromo- some are of primary importance in distinguishing the south- ern Guatemalan teosintes from maize, and to some extent from the other teosinte varieties. The occurrence of the gene Tu in only 15 percent of the backcross population, rather than in the 5O percent which might be expected from a random segregation, is apparently due to the low proportion of viable gametes which carry the maize allele. In fact, it would be necessary to assume a steril- ity of 41.1 percent, on the basis of the data in the present experiment, to account for the distribution of Tu and tu plants in the backcross population. As the segregation in this par- ticular population is 73 non-tunicate to 13 tunicate, 60 in- viable gametes carrying the Tu gene would be required to maintain an equality in number of gametes carrying the two alleles. 1G BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION Assuming that this particular population affords a rep- i resentative sample of the gamete production of the F1 hybrid, . it may be seen that the number of required inviable gametes (60), divided by the total gametes in the sample (73 + 13 + i 60), gives the percentage (41.1) of sterility necessary to account for the distribution of the Tu and tu genes. This, of course, is the minimum sterility required for such a distribu- tion, and each time that tu occurs in a non-functional gamete i, there would be an increase in the sterility required to account i; for the abnormal ratio of tunicate and non-tunicate plants. Therefore, since the fourth chromosome is apparently involved "i in a minimum of 41.1 percent of the sterility, and the observed sterility in these hybrids is approximately 50 percent, Drac- ’ tically all of the sterility may be accounted for by the abnor- mal behavior of the fourth chromosome. The relative deficiencies of the fourth chromosome genes i, in the backcross populations, as well as the linkages of these f same genes with sterility, suggest that the factor or factors i leading to this sterility are located very close to Tu, and be- tween Tu and sul. There is apparently some lack of homology '. between maize and Florida teosinte in this particular section ’_ of the fourth chromosome. And not only is this lack of homol- 3 ogy found between maize and Florida teosinte, but also be- ' tween the southern Guatemalan varieties and all other varie- ties of teosinte and maize so far studied. Apparently this = lack of homology between the two types of parental fourth A chromosomes leads to sterility in the F1 hybrids, and the 1 mechanism causing this sterility results in the formation of . non-functional gametes which carry a certain segment near 5 the Tu locus of the maize fourth chromosome. Further genetical and cytological studies of these hybrids, with particular emphasis on the fourth chromosome, should provide a better understanding of this characteristic chromo- A some structure of the southern Guatemalan varieties, and §, perhaps determine in what manner the chromosomes of these T varieties differ from the chromosomes of both maize and other y teosinte varieties. Summary Hybrids of the Mexican teosinte varieties with maize exhibit approximately normal fertility. However, the per- centage fertility of the Mexican teosinte-maize F1 hybrids § increases slightly with the use of the Chalco, Nobogame and . Durango varieties, respectively. One exception in the be- 1 FERTILITY RELATIONSHIPS IN MAIZE-TEOSINTE HYBRIDS 17 ¢havior of the Mexican teosintes is found in the F1 hybrid {involving the Nobogame variety; semi-sterile plants are some- times observed. A segregation of normal and semi-sterile plants in a 1 :1 ratio occurs in one of the (Nobogame teosinte 4» maize) x maize backcross populations, and it is suggested that a reciprocal translocation may be responsible for this ‘condition. Hybrids of maize with the Huixta variety of northern Guatemala exhibit relatively normal fertility, although the lpercentage is somewhat below that found in the Mexican teosinte-maize hybrids. In contrast, the F1 hybrid of maize fwith the Florida variety of southern Guatemala shows an iaverage pollen fertility of 49 percent. A study of nine back- cross populations involving the Florida teosinte variety re- iveals that the maize parent has a pronounced effect on the ;,degree of sterility. Linkage studies show that a definite ‘linkage exists between sterility and marker genes on chromo- Tsome 4. There is a marked deficiency in the backcross popu- Jations of the maize alleles of all fourth chromosome marker genes. » The behavior of the inter-varietal teosinte hybrids shows ‘that Florida teosinte produces hybrids of approximately 50 {percent fertility in crosses with the Mexican varieties, but that hybrids involving only the Mexican varieties are rela- itively normal in fertility. A consideration of all present information on maize- teosinte and inter-varietal teosinte hybrids indicates that the ‘southern Guatemalan teosinte varieties differ from the other teosinte varieties and maize in chromosomal homology. The Tesults of the sterility studies show that some factor or fac- Qtors on the fourth chromosome, located very close to Tu, and }b€tW€€I1 Tu and sul, account for most of the sterility in hy- ‘brids involving these southern Guatemalan varieties. Appar- - ently the mechanism causing this sterility results in the forma- ition of non-functional gametes, which carry a certain segment of the maize fourth chromosome near the Tu locus. Acknowledgment I The writer wishes to acknowledge the criticism and in- Tterest of Dr. Paul C. Mangelsdorf, formerly vice director of the Texas Agricultural Experiment Station, now director of Tithe Botanical Museum of Harvard University, under whose direction the investigation was conducted. 10. 11. BULLETIN 730, TEXAS AGRICULTURAL EXPERIMENT STATION Literature Cited Arnason, T. J. Cytogenetics of hybrids between Ze? Mays and Euchlaena mewicana. Genetics, 21:40-60. 1936i: Beadle, G. W. Studies of Euchlaena and its hybrids Wit Zea. I. 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