Range-wide genetic analysis reveals limited structure and suggests asexual patterns in the rare forb Senecio macrocarpus

Genetic diversity and recombination underlie the long-term persistence and evolution of species and are strongly influenced by population size, breeding system and plant longevity. Here, we study genetic structure in the rare Senecio macrocarpus in southeastern Australia to guide current conservation practices. Thirteen neutral microsatellite markers and two chloroplast regions were used to survey the 20 known S.macrocarpus populations and one sympatric S.squarrosus population, a morphologically similar species. All markers showed severe excess or deficit of heterozygotes and linkage disequilibrium was significant. Microsatellite markers revealed 100 multi-locus genotypes (MLGs) from 523 S.macrocarpus individuals and a further 4 MLGs from 27 S.squarrosus individuals. MLGs varied in frequency and distribution. At the extremes, one MLG was found 108 times across the sampling region and 66 MLGs were found once. The MLGs of all 38 seedlings genotyped were identical to their seed parents implying an asexual origin. Chloroplast regions showed little variation within S. macrocarpus but differed from S. squarrosus. Chromosome counts for S. macrocarpus revealed the same ploidy level as S. squarrosus (2n=6x=60) and pollen-ovule ratios were typical of erechthitoid Senecio species showing self-compatibility. Results suggest that establishment of small populations occur primarily from one extensive source population with indications that both apomixis and selfing may be contributing to its reproduction cycle. We suggest that this species may contribute to future evolutionary processes despite limited genotypic variation and restricted distribution. Its conservation will safeguard evolutionary processes that might occur through occasional outcrossing and hybridization events between sympatric species.