Institute of Cytology and Genetics, Russian Academy of Sciences
Southern Plains Range Research Station, USDA-ARS
It is quite obvious that in species with different types of reproduction through asexual seeds contributions of these factors are not the same. For that reason apparently itís more correct to discuss experimental results only in application to the object on which they were obtained in order that these might not be exaggerated more than the method used allows.
The present report is the result of studying the offspring of corn-gamagrass hybrids with different ratios of parental genomes (the pedigrees were published earlier in Sokolov et al., Russian Genetics 34 : 499-506, 1998), produced from backcrossing F1 (2n=56; 20Zm + 36Td) by corn and very rarely spontaneously obtained hybrids with doubled genomes: 1) 2n = 2 x 39 (30Zm + 9Td) = 78; 2) 2n = 2 x 38 (20Zm + 18Td) = 76.
Also, new crosses of corn with gamagrass were made and F1 hybrids were obtained that were backcrossed by corn with the purpose of analysing for segregation between nonreduction and parthenogenesis in gamagrass by family. Twenty three BC1 families were studied in all.
In Table 1 the results of segregation among the offspring of the corn-gamagrass hybrids are cited. From the data presented itís quite evident that the number of sexual offspring (BIII + BII) increases with an increase in the corn portion of the genome of the hybrids (the sexual parent) relative to the number of genomes introduced into them by gamagrass (the apomictic parent). It need be noted that the effect of the increase in the number of corn chromosomes on suppression of nonreduction (BII-hybrids) and parthenogenesis (BIII + BII- hybrids) is not the same. As we can see, parthenogenetic development may not be realized even with a 1:1 ratio of the genomes (F1 hybrids).
Table 1. Segregation in the offspring
of corn-gamagrass hybrids with different ploidy levels for apomeiosis and
|Number of apomicts||Number of BIII-hybrids||Number of BII-hybrids||Total|
|F1 2n=56 (20Zm + 36Td)||98||7||0||105|
|BC 2n=38 (20Zm + 18Td)||177||4||0||181|
|BC 2n=39 (30Zm + 9Td)||132||13||0||145|
|BC 2n=59 (50Zm + 9Td)||21||13||2||36|
* The genome of this line has one unusual chromosome 6 from corn carrying an extra NOR on the long arm in addition to the regular NOR on the short arm. It has been previously reported as a Mz -Tr translocation but as the analysis of spacer regions was not made such an affirmation is unproved (Kindiger, B et al., Genome 39 : 1133-1141, 1996). Besides, the line carries two different size telocentric chromosomes. The line is derived from a 39-chromosome line (30Zm + 9Td), so phenotypically and by its hybridological behavior it is close to the hybrids having this genome.
**BIII-hybrid produced from pollination of the 40-chromosome line with tetraploid corn.
The development without fertilization of egg-cells may not be realized even in lines with a 2 : 2 ratio of the genomes (F1 hybrids) and we observed this in 7 cases among 105 plants. Such a proportion (about 10%) of BIII offspring holds in the hybrid lines up to a 5Zm : 0.5Td ratio of the genomes when their number sharply increases to 35%. For a significant increase in a proportion of egg - cells with a reduced chromosome complement (BII-hybrids) a tenfold difference in a ratio of the parental genomes is needed but even in that case their number is by an order less than that of fertilized unreduced egg- celles(BIII-hybrids).
These results suggest the independent penetrance of the two constituents of apomixis as well as a difference in the number and quality of genes involved in their control. The apomeiotic constituent presented in Table 1 is programmed sufficiently rigidly and realized as dominant even with a multifold difference in the number of the genomes in favour of the sexual parent. At the same time parthenogenesis exhibits incomplete penetrance even in F1 hybrids and further is highly labile and decreases inversely to increases in the ratio of corn genomes to gamagrass genomes.
A small sampling of results is presented in Table 2. The ability to obtain offspring from these hybrids is complicated enough by reason of their very high female sterility. High-productive tillering is characteristic of both, so we pollinated about 700 flowers of the 78 chromosome plants with tetraploid and diploid maize pollen and obtained 34 very shrunken kernels. All these were from pollination with the commercial hybrid ICI (2n=20). In total only 15 of them gave us plants. The results concerning the second plant were taken from the work of our laboratory published earlier (Yudin and Lukina, Proc. Acad. Sci. (Russia), : 273,#5, 1246-1248,1983). These hybrids were also actively pollinated and set 7 kernels from pollination with hexaploid corn (2n=6x=60).
Table 2. Segregation in the offspring
of the doubled corn-gamagrass hybrids.
|BC 2n=78; 2 (30Zm + 9Td)||0||14||1||15|
|BC 2n=76; 2 (20Zm + 18Td)||7||0||0||7|
Itís noticeable that 14 plants from the 15 obtained from the 78-chromosome form, turned out to be dihaploids and the other was a BII-hybrid., Unlike the 39-chromosome apomicts, which were always sufficiently homogeneous morphologically, the plants in question markedly differed from one another both in tillering degree (1 to 8) and in number of ears, character of their placement and development. In the given family we observed many off-types noticed earlier as being rare autosegregants in the 39-chromosome lines. It is possible that this is a consequence of epigenetic marking realized under the meiotic development of egg- cells.
Weíll especially stress that isogeneity under the doubling of the small gamagrass complement in the 78-chromosome plants leads to the normal proceeding of meiosis and formation of reduced egg- cells which develop in the main parthenogenetically and we observe "dihaploid" offspring. This is affirmation of independent penetrance of hereditary structures responsible for apomeiosis and parthenogenesis as in the given case the latter is realized not after nonreduction but in an inverse variant after meiosis.
The second plant in Table 2 (the 76-chromosomes) produced only apomictic offspring. When comparing its hybridological behaviour with the preceding case one may suppose that the action of two haploid complements of gamagrass chromosomes differs from that in gamagrass with twice the complement of 9. Perhaps the effect of 60 corn chromosomes is also stronger in the 78-chromosome plants than the 40 in the 76-chromosome plants.
And finally the data presented in Table 3 generalize the results from backcrossing a 46-chromosome F1 hybrid by diploid corn. Most BC1 families (15 of 23) proved to be apomicts, that is, like the mother plant, they had 46 chromosomes. Another group (7 families) were represented mainly by apomictic offspring but in addition they had 1 to 3 BIII-hybrids. In that case, as well as in those considered before, parthenogenesis does not have 100% penetrance, though the ratio of the genomes 1 : 2 is in favour of the apomictic parent.
Table 3. Segregation in gamagrass (2n=72)
for parthenogenesis trait.
One family (363) was represented by BIII-hybrids only, i. e. parthenogenesis is absent from this form.
All the results present evidence for the independence of the control of apomeiosis and parthenogenesis and for the possibility of segregation in gamagrass. Besides, based on these results, one may suppose that the number of genes controlling development without fertilization exceeds the number of genes controlling nonreduction and their penetrance depends on many genotypical circumstances and external factors.
The authors express deep appreciation
to Dr. J. G. Carman for fruitful discussion. The research was supported
by the Netherlands Organization for Scientific Research Grant No. 047.007.019
and Russian Foundation of Basic Research Grant No. 97-04-49301.
Return to the MNL 74 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page