Plant & Animal Genomes XI Conference Plant & Animal Genomes XI ConferenceJanuary 11-15, 2003
Town & Country Convention Center
San Diego, CA
Workshop: Apomixis
Antennaria consists of 32 sexual (primarily diploid) species distributed among six monophyletic clades. Agamospermy occurs in one clade only, the Catipes, which contains 17 of the 32 sexual species. Factors differentiating the Catipes from other clades include 1) strong habitat speciation along ecological (edaphic, elevational, latitudinal, hydrologic) gradients, 2) strong sexual dimorphism (dioecy), and 3) extensive polyploidy. Morphological, ecological, cytogenetic, molecular and embryological studies suggest that the five agamic complexes occurring in the Catipes are of recent origin and evolved from extant sexual species through multiple interspecific hybridizations and polyploidization. The coexistence of apomicts and their immediate sexual progenitors makes the Catipes ideal for studying the origins of apomixis. In this context we have created and are testing an evolutionary model consisting of three steps: initial expression, penetrance enhancement and genetic stabilization. Accordingly, facultative apomixis arises in hybrids produced from sexual parents that are divergent in onset and duration of certain phases in the embryo sac development sequence (ESDS). Penetrance is enhanced in subsequent sexually-derived generations when rare segregants arise in which the asynchronous expression of certain ESDS components is accentuated. Since apomixis requires rare combinations of alleles at multiple loci, it is readily disrupted when gene flow and recombination are unrestricted. Genetic stabilization (shielding from recombination) initially occurs through polyploidization. Elaborate stabilization mechanisms sometimes arise in ancient apomicts where linkage disequilibrium is enforced by unusual genetic mechanisms and/or more robust forms of karyotypic heterozygosity.