Apomixis in Tripsacum: Comparative mapping of a multigene phenomenon

A relationship has been established between the expression of apomixis in n atural polyploids of Tripsacum dactyloides and fertility as measured by per cent seed set. Thus, fertility may be reliably used as a defining phenotype for apomixis when scoring the progeny from diploid (2n = 2x = 36) x tetrap loid (2n = 4x = 72) crosses in Tripsacum. By exploiting the relationship be tween apomixis and fertility, as defined by seed set, analyses were perform ed on a set of related second-generation triploid populations segregating f or apomixis. These populations were derived from sexual (diploid) x apomict ic (tetraploid) crosses. Six out of 25 genome-dispersed restriction fragmen t length polymorphism (RFLP) markers co-segregate with fertility. Five of t hese markers were previously reported and include: php20855, tda48, tda53, umc62, and umc83, and are linked to Tripsacum genetic linkage groups F, I, H, L, and A, respectively. Significantly, we report here the syntenic relat ionships of the maize chromosome intervals to Tripsacum that segregate for numerous meiosis-specific and fertility-associated genes. Utilizing RFLP lo cus comparative mapping based on conservation of chromosome (genic) regions between related species, it may be concluded that the genes controlling fe rtility have been preserved in both Tripsacum and maize. A sixth marker, um c166, has also been shown to co-segregate with fertility and is conserved i n both grass species. Specifically, umc166 is linked to Tripsacum linkage g roup D and, by syntenic comparison, to the short arm of maize chromosome 5. Encoded within this marked interval is the gene Ameiotic1 (Am1) whose func tion is required for the initiation of meiosis in both micro- and megaspore mother cells and whose absence of expression in the female is, in all like lihood, a prerequisite for the expression of apomixis.