Disomic inheritance, suppressed recombination, and allelic interactions govern apospory in buffelgrass as revealed by genome mapping
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Molecular tools have not identified the gene(s) governing apomixis nor have they been used to successfully transfer the trait to important, sexually reproducing food crops. Several molecular studies addressing apomixis in grasses have used interspecific and intergeneric hybrids. The failure to recover specific F1genotypes from these wide crosses can be caused by unfavorable interactions between the gametes, zygote, embryo, endosperm, and/or maternal tissue. These interactions can eliminate recombinant genotypes with valuable information towards linkage analyses of the trait. Buffelgrass [Pennisetum ciliare (L.) Link syn. Cenchrus ciliaris L.], a polymorphic species with interfertile apomictic and sexual genotypes, offers an opportunity to genetically map apomixis by means of intraspecific hybrids with euploid genomes. This study reports a linkage map of the apospory region in buffelgrass. Apospory, classified by progeny testing and cytologically observing megagametophytes, mapped to a single locus in the apomictic parent's genome. Two buffelgrass cDNAs (pPAP3A07 and pPAP8C08) and three previously reported apospory markers (UGT197, QH8, and OPC4) were tightly linked (1.4 centimorgans, cM) to the trait. As a tetraploid species (2n = 4x = 36), four copies of each chromosome are expected in buffelgrass. A single homolog and two homeologs were identified for the chromosome carrying apospory, indicating the formation of two bivalents during meiosis and the disomic inheritance of apospory in buffelgrass. Allelic bridges between the parents revealed suppressed recombination in the apospory linkage group. Segregation distortion between a marker on the sexual parent's homolog to the apospory linkage group and a marker on a separate maternal linkage group suggested specific allelic combinations in female gametes affect offspring survival in buffelgrass.
