Chapter 5 Genetic Analysis of Apomixis - cimmyt

Goottk AIaIy,Is .f Apolllilis 69Brachiaria did not totally prevent selfing.Techniques have been published foremasculating small flowered grasses (Burson1985,1992; Richardson 1958).4) Male gametocide. Young inflorescences ofPennisetum ciliare were sprayed with a malegametocide (Bashaw and Hignigh 1990;Hignight et al. 1991).5) Self incompatibility. Self incompatibility ofsexual female lines has been used to advantagein Taraxacum (Richards 1970), Paspalum notatum(Burton and Forbes 1960), and Hieracium(Gadella 1987). Dujardin and Hanna (1989)used male sterile pearl millet (Pennisetumglallcum) as a female parent. Selfincompatibilityis incomplete in guineagrass(Panicum maximum); the degree of crossfertilization depends upon the procedurepracticed for isolation (Savidan et al. 1989;Savidan 1990b). Because genotypes can varyin degree of spontaneous selfing, it may benecessary to use control tests to establishreliability of each female parent (Matzk 1989;Valle et al. 1991).Reciprocal CrossingReciprocal crossing detects nuclear andcytoplasmic maternal effects (includingmatrociiny). The apomixis gene(s) has such apowerful maternal effect that geneticists havebeen discouraged from using this technique,but apomixis need not be a deterrent to its use.It is feasible to use one or two facultativeparents in the crossing scheme (Savidan 1981).Nogler (1984b) conducted reciprocalbackcrosses with a sexual diploid genotype asthe male or female in pairings with facultativelines. Jassem (1990) used reciprocals in beets.Creating Tetraploid ParentsIt is best if both parents are at the same ploidylevel. The tetraploid level seems to be thenatural milieu for expression of apomixis,whereas apomixis is rarely confirmed indiploids. If fully sexual tetraploids are notavailable, they may be produced by variousstrategies. Colchicine treatment of sexualdiploids has created sexual tetraploids usedin hybridizations (Burton and Forbes 1960;Richards 1970; Savidan 1981; Miles and Valle1991; Valle et al. 1991). Leblanc et al. (1995a)treated embryogenic calli of sexual diploidTripsacum with colchicine to inducechromosome doubling; the regeneratedtetraploid plants reproduced sexually.However, when a sexual line of PaspalumheXilstachium was doubled, the tetraploid wasfacultatively aposporous (Quarin and Hanna1980). Asker (1967) started with an apomicticdiploid (possibly a dihaploid?) biotype ofPotentilla argentea and obtained a partiallysexual tetraploid. Thus colchicine doublingmay reveal latent capacities for apomixis orsexuality.Tetraploids can be created by hybridization.Harlan et al. (1964) selected a completelysexual tetraploid of Bothriochloa grahamii froma cross between two facultatively apomictictetraploids. Savidan's (1981) crosses offacultative tetraploids of genotype Aaaayielded sexual:aposporous (S:A) progeny inthe ratio of 1:3, indicating that the sexualprogeny were genotype aaaa. The geneticanalyses of Harlan et al. (1964) also used asexual tetraploid Dichanthium annulatumaccession that originated from a BIllhybridization ofan emasculated diploid sexualplant with a tetraploid male. Burton andHanna (1986) grew diploid sexual Pensacolabahiagrass (Paspa/um notatum) in isolation withan apomictic tetraploid to produce a triploidBmhybrid. Open pollination of the triploidwith Pensacola bahiagrass yielded Bill hybridsat the tetraploid level. These facultativelyapomictic lines could be used as maternalparents in crosses and selfs to create sexualtetraploids. Quarin (1992) successfully used asimilar scheme.