Diplospory and Parthenogenesis in Sexual x Agamospermous (Apomictic ) Erigeron (Asteraceae) Hybrids

Segregation for asexual seed production was evaluated for 130 experimental F1 hybrids resulting from a cross between diploid (2n=18) sexual Erigeron strigosus and triploid (2n=27) agamospermous Erigeron annuus. Paternity of hybrids was documented using 13 RAPD markers. The distribution of F1 chromosome numbers is bimodal, centering on diploid and triploid ploidal levels but with underrepresentation of diploids. Diplosporous versus meiotic megagametophyte development was ascertained microscopically for >/=100 ovules per plant. Diplospory ranges from 0% to 100% among all progeny but is uniformly low (0%-3%) for 17 diploid hybrids. The inheritance of diplospory in Erigeron appears to be best explained by a one-locus-two-allele polysomic model with selection against gametes homozygous for diplospory. Parthenogenesis, estimated via seed counts, ranges from 0% to 60% and apparently is contingent upon diplospory, as seed production was absent or very low in predominantly meiotic hybrids. However, the absence of parthenogenesis in many highly diplosporous hybrids indicates that these two aspects of agamospermous development are not strictly associated. The segregation of both diplospory and parthenogenesis in this population will permit further genetic dissection of these traits with molecular marker-based analyses.