Species with contrasting forms of rarity show differences in genetic structure

Background/Question/Methods Rare species often occur in populations that display very different spatial arrangements and this may, in turn, result in different spatial genetic structures. We described the spatial structure of two closely related taxa, Streptanthus glandulosus ssp. niger (Brassicaceae) and S. glandulosus ssp. secundus, both of which are naturally rare, habitat specialists found only on stressful serpentine soils. S. glandulosus ssp. niger exists as a single large population with relatively high local density and is Federally protected as Critically Endangered. Its sister taxa, S. glandulosus ssp. secundus, exists in very small populations (5 – 50 plants per population) scattered across a large geographic area and separated by distances that exceed seed dispersal distances. It is also protected as Rare. We quantified genetic diversity in the one population S. glandulosus ssp. niger and in all 11 known populations of S. glandulosus ssp. secundus using 10 polymorphic microsatellite markers.

Results/Conclusions

Allelic richness was low for S. glandulosus ssp. niger (A_R=6.37) but was even lower in S. glandulosus ssp. secundus (A_R=4.1). The effective number of alleles followed the same pattern (A_E=3.3 and A_E=1.77, respectively). Both expected and observed heterozygosity were higher for S. glandulosus ssp. niger (H_E=6.5; H_O=5.86) than for S. glandulosus ssp. secundus (H_E=4.9; H_O=3.77). Further, the degree of inbreeding was moderately high in S. glandulosus ssp. niger (F_IS=0.141) and pronounced in S. glandulosus ssp. secundus (F_IS=0.22).

Comparisons of population genetics have been made for rare species and their common relatives. We are unaware of any such comparison for species that display contrasting forms of rarity. We are exploring the implications of low genetic diversity and high inbreeding for these two taxa. Early results indicate that inbreeding depression may exist in S. glandulosus ssp. niger whose range has been reduced by >50% in the past 50 years due to housing development. Interestingly, S. glandulosus ssp. secundus, which has much lower levels of genetic diversity and higher levels of inbreeding, does not appear to be suffering any loss of fitness due to inbreeding. We attribute this to a long history of naturally small and patchy populations. This information will be useful for developing effective conservation plans for these taxa.