Chapter 5 Genetic Analysis of Apomixis - cimmyt

32 CWIts F. C,..haploids from normal cotton plants pollinatedwith semigamous pollen have been reported(Turcotte and Feaster 1963). In the absence ofsuch rare haploids, one is tempted to presumethat semigamy results from induction of asubset of zygotic behavior that preventskaryogamy but not plasmogamy. Ifsemigamywere found to function in both eggs andsperms, one would instead suspect a generaldefect in fertilization capacity.The variation from autonomous to pollinationdependentembryo induction in pseudogamysuggests that embryos can be induced in morethan one way. Perhaps one way triggers fullembryonic development, while anothermerely primes the egg (or sexual zygote) torespond to a stimulus from the dividingendosperm. In the latter case, the apomicticegg of an unfertilized ovule would degeneratewithout ever dividing, whereas in the formercase, the apomictic egg would eventuallydivide, even if it does not regularly do sobefore the primary endosperm nucleusdivides or before unfertilized embryo sacsdegenerate. Savidan (1989) has proposed thatbecause pseudogamous Panicum maximumundergoes such early induction andmaturation of the embryo sac, the egg cellcompletes its cell wall before pollination.While the formation of the egg wall isprogressive in grasses (Cass et al. 1986), it isnot clear that old eggs inevitably wouldcomplete their wall. In vitro fusion of egg andsperm protoplasts of maize leads to rapid cellwall formation (Breton et al. 1995, andreferences cited therein), suggesting thatinduction of zygotic behavior is responsiblefor the physical barrier to fertilization ofpseudogamous eggs.Autonomous parthenogenesis would seem torequire separate inductions of endospermdivision and either the "triggered"(Wikstroemia) or merely the "primed" (Crepis)condition. Yet separate inductions wouldimply that the genes for each could beseparated and that genotypes lacking theautonomous endosperm induction would befully pseudogamous.This is possible in generawhere instances of pseudogamy andautonomous parthenogenesis are known, e.g.,Poa and Crataegus, but it is inconsistent withunreduced autonomous parthenogenesis inthe absence of pseudogamy in apomicticCichoreae (Taraxacum, Chondrilla, Crepis, andIxeris).Genomic imprinting of gametic nuclei isanother consideration in the developmentalinterpretation ofapomictic embryogenesis. Inthe Polygonum-type, the endosperm has a 2:1maternal:paternal genome ratio; deviationsfrom this ratio frequently cause endospermabortion in interploidy crosses. InsemigamousCooperia, the reduced sperm nucleus fuseswith both unreduced polar nuclei (Crane1978), resulting in a 4:1 maternal:paternal ratioin the endosperm, which functions adequatelyfor seed germination in this genus. Inpseudogamous Rtmunculus, both sperm nucleican fertilize the central cell, resulting in a 4:2maternal:paternal ratio and normalendosperm function (Nogler 1972). Inpseudogamous Crataegus, the polar nuclei donot fuse with each other, and a sperm fuseswith only one polar nucleus, resulting in thenormal 2:1 maternal:paternal ratio (Campbellet al. 1987). The situation in autonomousparthenogenesis is not clear, because thenumber and biochemical effect of imprintedloci is not obvious. Imprinting of a tubulinlocus (Lund et al. 1995) and the dzrl locus(Chaudhuri and Messin 1994) has beendocumented in developing maize endosperm,and it is possible that the number and sexspeCificityof imprinted loci vary greatlyamong plant species. What matters here is thenumbe~ of loci that orchestrate the imprintingpattern. Since the evolution of apomixis iseasier if fewer loci control it, one might expect