Perspectives of developing apomixis in maize
--Sokolov, VA, Lukina, LA, Khatypova, IV
Creation of apomictically reproducing maize hybrids makes it possible to obtain high yields with the fixed heterosis effect characteristic of the F1 with minimum expenditures for seed-farming. For this purpose D.F. Petrov proposed the adoption of genes controlling asexual reproduction in a wild maize relative - Tripsacum (Petrov, Nauka, 1964). At his laboratory hybrids of these plants with a diverse ratio of the parents' genomes and exhibiting apomictic reproduction type were obtained (Petrov et al., Genetika 3:58-71, 1968). Due to a number of negative qualities received from Tripsacum they could not be used in practical selection. However, in the apomictic progeny with the highest frequency plants were revealed, developing from reduced gametes either apomictically or in a sexual way. Among such plants selected were forms carrying 1, 2 or 3 chromosomes of Tripsacum along with 20 chromosomes of maize (Petrov et al., 1984). Later several apomictic lines of maize were reported, isolated thanks to crossover introgression into its genome of appropriate genes from Tripsacum chromosomes (Petrov et al., Dokl. Akad. Nauk 281:509-512, 1985; Fokina, N. I. Vavilov VOGIS 5th Congress Part 2:217-218, 1987). However by hybridization analysis these lines were shown not to carry the apomixis character (Yudin and Sokolov, Dokl. Akad. Nauk 309:219-222, 1987).
The analysis of the results obtained by 1987 led us to setting ourselves the task of establishing the role of separate Tripsacum chromosomes in cytogenetic control of apomixis in the hybrids. The genomic complements of the apomictic maize x Tripsacum hybrid forms known by now have a different number of Tripsacum genomes. But in all cases 18 chromosomes were the least number ensuring ability for apomictic development and at the same time carrying the negative characters of the wild parent. In this connection, as the first step in the way of apomictic maize creation, we tried to increase the frequency of occurrence in the progeny of forms with a reduced number of Tripsacum chromosomes by changing a ratio of genomes of maize and Tripsacum in the hybrids. The report presented here contains the results of this experiment.
As initial material, 38-chromosome forms were taken, obtained earlier at our laboratory by fertilization of reduced egg-cells of 56-chromosome plants with pollen of diploid maize and including in their genomic complement two sets of maize chromosomes and one set from a tetraploid (2n=72) apomictic Tripsacum variety. These lines for a long time year after year have been producing 97 to 100% of apomictic progeny, evidencing that the haploid set of Tripsacum chromosomes is enough to control this character.
From our experience of work with the hybrids we knew that an increase in maize : Tripsacum genome ratio resulted in an increase in probability to develop progeny with chromosome reduction. So to solve our task 58-chromosome unreduced hybrids were taken, obtained by pollination of 38-chromosome plants with pollen of tetraploid maize. The genomic complement of these hybrids includes 40 maize chromosomes and 18 Tripsacum chromosomes, corresponding to a 4:1 ratio of the haploid sets. The high level of reduced progeny in such plants permitted us to hope for successful recovery among these of maize addition lines, where to maize genomes were added 1 to 18 Tripsacum chromosomes in different combinations. The total number of possible variants making 257,442 in this case would be impossible to examine cytologically. Only recovery of apomictic plants with a reduced number of Tripsacum chromosomes considerably simplified our task. In the progeny of the 58-chromosome unreduced hybrids we managed to isolate 39-chromosome apomictic plants in which to maize genome sets were added only 9 chromosomes of Tripsacum, reducing the number of combinations of variants to 511. By means of obtaining the 39-chromosome forms we managed to pass from the 56-chromosome hybrids having 36 chromosomes of Tripsacum in their complement to hybrids with 9 chromosomes of Tripsacum .
From these sufficiently stable 39-chromosome apomicts, by pollinating them with pollen of tetraploid maize, a further increase in maize: Tripsacum genome ratio was obtained in unreduced 59-chromosome hybrids. The frequency of such hybrid occurrence is about 0.5 %. To enhance the manifestation of the sexual reproduction character 79-chromosome hybrids (70 maize chromosomes + 9 Tripsacum chromosomes) were derived. From these we succeeded in developing plants with an imbalanced set of maize chromosomes (i.e. not divisible by the haploid set) and with an addition of separate chromosomes of Tripsacum (from 3 to 5). These plants with a high female sterility yield fertile pollen. We have so far failed to isolate apomicts among them by reason of small volume of this material. However, high tendency to sexual reproduction of the 79-chromosome hybrids producing more than 50 % of reduced progeny makes it really possible to increase this volume. Besides, as we deal with Antennaria-type apomixis due to partial meiosis with the genomic complement imbalanced in the 39-, 59-, 79-chromosome plants, crossover exchange of sites is possible between maize and Tripsacum chromosomes that are partially homeologous owing to their distant relationship (Koltunow, Plant Cell 5:1425-1437, 1993; Galinat, Ann. Rev. Genet. 263:1598-1600, 1971). If in such a process a chromosome participates carrying gene(s) for apomixis its transition under the control of a maize centromere is possible.
In addition to the above - mentioned 39-chromosome plants with a high level of apomictic progeny (95 - 100 %), different 39- and 49-chromosome hybrids (30Mz + 9Tr; 40Mz + 9Tr) were developed by us. These lines were derived from the only parthenogenetic reduced hybrid originating from a 58-chromosome plant (40Mz + 18Tr). It should be noted that this plant (20Mz + 9Tr) was highly sterile and, when pollinated, it yielded one single plant with 39 chromosomes (30Mz + 9Tr), serving as the ancestor of the above lines. It should be stressed that the chromosome complement in these lines does not coincide with that in the formerly developed hybrid. For that reason they produce only 10-15% of apomictic progeny and are partially male sterile. These results confirm John Carman's hypothesis of polygenic control of apomixis and in that case the approach to the creation of apomictic maize has been revised by us about which a separate report will be made on the basis of the new experimental results (Carman, Biol. J. Linnean Soc. 61:51-94, 1997).
The work was done (partially) at the expense of the "Priority directions
of genetics" grant.
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