Chapter 5 Genetic Analysis of Apomixis - cimmyt

........UpoonkIk Spede. 145The mode of reproduction of an independentset of 107 first generation Brachiaria hybridswas determined by progeny tests and byembryo-sac analysis in Colombia (Table 10.4).Embryo-sac analysis determined that 56 of theplants were sexual and 51 were apomicts(Miles and Valle 1991), a finding that agreeswith the proposed hypothesis of simpleinheritance. Although interspecific crossesmay not be ideal for studying inheritance ofapomixis, work on the agamic complexformed by B. ruziziensis, B. brizantha, and B.decumbens indicates simple genetic control forapomixis. Sexual x apomictic crosses releaseda large amount of phenotypic variation in theprogeny (plant morphology, growth habit, andflowering time).Progeny tests of the 107 open pollinated, firstgenerationinterspecific hybrids were alsoused to assess reproductive behavior (Milesand Valle 1991). Seeds harvested fromindividual plants were sown in five-plantplots, in one to four replicates. The mode ofTable 10.4 Comparison between progeny test andembryo-sac analysis for determination of mode ofreproduction for first-generation interspecificBrochiorio hybridsRate ofMode of reproduction sexuality (%)embryo-sacprogeny-test analysisembryo-sacanalysis54 hybrids sexual sexual37 hybrids apomictic apomictic 10 -734hybrids unclassified apomictic 7-832hybrids unclassified sexualFacultative apomicts first dassified as sexual541-03 sexual apomictic 10544-04 sexual apomictic 50549-02 sexual apomicfic 17554-02 sexual apomictic 63554-03 sexual apomictic 77683-0 I sexual apomictic 52687-01 sexual apomictic 70693-02 sexual apomictic 47694-07 sexual apomictic 43702-06 sexual apomictic 30reproduction of the mother plant was inferredfrom the relative uniformity or heterogeneityof the open pollinated progeny. The resultswere later compared to microscopicexamination of embryo-sac structures of thehybrid mother plants. The two methodsagreed closely, except for ten of the progenies,in which the degree of sexuality (determinedby embryo-sac analysis) ranged between 10and 77%. (Table 10.4). The degree of effectivesexuality as detected by the progeny test wasnot closely associated with the proportion ofsexual embryo-sacs observed microscopically.Whereas facultative apomictic hybrids with 10or 17% sexuality produced heterogeneousprogenies, other hybrids, in which sexualembryo-sacs were observed in up to 73% ofthe progenies, appeared to behave as obligateapomicts in the test. It is unclear what factor(s)con tribu tes to determining effectivereproductive behavior. Elucidation of themechanism of apomixis might help explain itsexpression under different circumstances.At CIAT and the Institute for Grassland andEnvironmental Research (IGER),Aberystwyth, Wales, U.K., a molecular markerfor the apomixis gene(s) is being sought, whichcould prove potentially useful for determiningreproductive mode. Pessino et al. (1997), usinga bulk segregant analysis and RFLPs andRAPDs, were able to identify molecularmarkers cosegregating with apomixis in asmall' (n = 43) Brachiaria F1 population. Twoclones (umc147 and umc72) belong to aduplicated linkage group that maps to thedistal part of maize chromosome-1long armand chromosome-5 short arm. Another,(OPC4), previously reported as a potentialmarker for apospory in Pennisetum, alsocosegregated well in Brachiaria. In later work,Pessino et al. (1998), using RFLPs and AFLPs,generated a complete map for the region inmaize chromosome 5, identifying at least twomarkers closely linked to the apospory region.