QUANTITATIVE ASSESSMENT OF MEGASPOROGENESIS FOR THE FACULTATIVE APOMICTS ERIGERON ANNUUS AND ERIGERON STRIGOSUS (ASTERACEAE)

Premise of research.Genetic and morphological variation in apomictic plant taxa is attributed in part to facultative apomixis and autosegregation, that is, alternative pathways by which apomictic plants may occasionally generate novel genotypes. Direct characterization of reproductive development in apomicts is infrequent, however. Apomictic polyploid Erigeron annuus and Erigeron strigosus (Asteraceae) are widespread in North America and exhibit high levels of morphological variation that are consistent with alternative reproductive modes.Methodology.To investigate reproduction in these taxa, 31 accessions of E. annuus and 42 accessions of E. strigosus were grown to maturity in a common greenhouse. Chromosome number and seed set were determined, and megasporogenesis was evaluated using differential interference contrast optics (>26,000 ovules were evaluated in this study).Pivotal results.Seed set among plants was highly variable (14.0%-95.5%; mean = 70.8%), implying that for many plants a substantial portion of ovules do not commit to apomictic development. Surprisingly, microscopic analysis showed that the frequency of diplospory is uniformly high (average similar to 95%) across genotypes, despite differences in seed set. Alternative development was observed for most genotypes at low frequency, including meiotic reduction via tetraspory (2.07% vs. 0.54% of ovules in E. annuus and E. strigosus, respectively), generation of micronuclei apparently via nondisjunctive autosegregation (2.70% vs. 1.94%), and trispory (0.55% vs. 0.29%).Conclusions.First, differences in seed set among apomictic genotypes must be primarily due to developmental failure subsequent to the diplosporous division of the megasporocyte, rather than to differences in diplospory rates per se. Second, diplospory is commonly incompletely penetrant, and a diversity of alternative modes occurs at low frequency.