Chapter 5 Genetic Analysis of Apomixis - cimmyt

Apomi.is aod rio. Maoage...., of Ge..'k Dlnnlty 19In model 2, apomixis is active in plants withthe Aa association of alleles. The aa genotypesare sexual. If Rn is the frequency of aagenotypes (sexual) and Qn is the frequency ofAa genotypes (apomictic), frequencies in thenext generation (n + 1) will be R n+ 1 = R (1-1 / n2Qn)' and Q n+ 1 = Q n (1 + 1/2R n )· In this case,the apomixis allele, A, diffuses in thepopulation as1 + 1/2Rn >1 and Qn + 1>Qn.We can use this model to define conditions ofequilibrium between sexual and apomicticforms if a differential fitness exists between thetwo forms. With a fitness of 1 + S for the an and1 for the Aa, the frequency changes fromgeneration n to generation n + 1 are as follows:R n+ 1= R n (1-1/2Qn)·(1 + s)/(l + sR n + sR n 2)Q n+ 1 =Qn(1 + 1/2R n ).1/(1 + sR n + sR/).In this case, equilibrium between sexual andapomictic forms will be reached for s = 1/1 +R. Initially, when apomixis starts to beestablished in a population, R is close to 1, andequilibrium can be reached with s values closeto 0.5. The fitness advantage of the sexual formsin relation to the apomictic forms has to be atleast 1.5:1 to reach the equilibrium. Onceapomixis is widely established, R is lower, andequilibrium will be reached only with highers values. In the extreme case of Q close to 1,equilibrium will be reached with s values closeto 1. In this instance, sexual forms will have toproduce twice as many seeds as apomicticforms to survive in the successive generations.If model 2 applies to apomictic varieties,transfer of apomixis to landraces could beaccomplished according to the process foundin model 1; but the transfer will take longer (atleast one more generation) because the firstgeneration will be made from Aa x aa crossesproducing Aa and aa genotypes, not from AAx aa crosses, which produce only Aa progeny.Conservation of diversity in the apomicticlandraces will depend, as in the former model,on the rate of residual sexuality, k.In model 3, apomixis is active only in plantsthat are homozygous for the recessive allele s.In this case, 55 (sexual) has a frequency of Pn,5s (sexual) has a frequency of 2Qn, and ss(apomictic) has a frequency of Rn. Using thismodel, it can be shown (Pemes 1971) that thefrequency of 5 behaves as follows:Pn + 1 + Qn + 1 = (Pn + Qn)(l - 1/2Rn)The frequency of 5 is reduced from onegeneration to the next, as 1 - 1/2Rn is alwayslower than 1.If the genetic control of apomixis follows thismodel, then transfer of apomixis will requireat least two generations. The pathway totransfer can be imagined as follows:1st generation: 55 [sexual] female x ss[apomictic] male =5s [sexual]2nd generation: 5s [sexual] female x ss[apomictic] male = 5s [sexual] + ss [apomictic]3rd generation: 5s [sexual] + ss [apomictic] xss [apomictic] or 5s [sexual] + ss [apomictic] =5s [sexual] + ss [apomictic] or 55[sexual] + 5s[sexual] + ss [apomictic]The apomixis gene can diffuse within thepopulation through backcrossing betweenplant& from the first generation and the donorvariety as male parent. In order to haveapomixis transferred within a reasonabletimeframe, the donor must be used as the malevariety of each generation. After severalbackcrosses, the local variety will betransformed to an apomictic variety, but it willbe almost identical to the donor variety.Therefore, if apomixis is active only whenrecessive alleles are present, it will be difficultto transfer apomixis to landraces while at thesame time maintaining the original traits ofthese landraces. It would require (i) the use of