Population genetic structure of a California endemic branchiopod, Branchinecta sandiegonensis

Regional structuring of genetic variation in short-lived rock pool populations of Branchipodopsis wolfi (Crustacea: Anostraca)

“…Across all populations studied, the level of amongpopulation genetic differentiation (F st =0.291) is higher than local differentiation in the freshwater anostracans Artemiopsis stefanssoni (F st =0.075) (Boileau et al 1992) and Branchinecta coloradensis (F st =0.124) (Bohonak 1998), but comparable to local differentiation in B. paludosa (F st =0.360) (Boileau et al 1992) and much lower than in a set of B. sandiegonensis populations (F st =0.657) sampled on a spatial scale of up to 50 km (Davies et al 1997). In obligately sexual brine shrimps, the reported levels of population differentiation varied between F st =0.24 in Artemia franciscana and F st =0.12 in A. salina (Abreu-Grobois and Beardmore 1982;AbreuGrobois 1987), whereas only a weak overall genetic differentiation was observed among nine A. sinica populations (F st =0.02-0.25) separated by as much as 1000 km the measured correlation between average pool temperature and allele frequency at three loci (GPI, PGM, AAT) at the same time is the result of selection, as this association is usually only manifested at the GPI locus (Riddoch 1993).…”

“…Significant deviations from H-W equilibrium due to heterozygote deficiencies were also found in populations of Branchinecta sandiegonensis, another anostracan occurring in short-lived pools (Davies et al 1997). These observations on freshwater anostracans from ephemeral pools are in contrast to patterns in temporary cladoceran populations that generally are in H-W equilibrium (Hebert 1987b).…”

“…Adult B. wolfi population densities vary in space and time, and because pools often hold less than a few hundred animals, it may seem likely that drift could be rapid (Boileau and Taylor 1994). However, delayed hatching from the mixed egg bank may reintroduce lost alleles from previous years and stabilise allele frequencies (Hairston and DeStasio 1988;Boileau and Hebert 1991;Davies et al 1997), hence reduce the likelihood that genetic drift may occur rapidly.…”

“…This has been explained by persistent founder effects (Hebert 1974a(Hebert , 1987aBoileau and Hebert 1991;Boileau et al 1992;Berg and Garton 1994;Boileau and Taylor 1994), temporally fluctuating selection (Lynch 1987), and local adaptation (Hebert 1974b;Mort and Wolf 1986;De Meester 1996). Despite significant differentiation in allele frequencies, the association between genetic and geographic distance in zooplankton populations is weak at best (Innes 1991;Lynch and Spitze 1994;Davies et al 1997;Vanoverbeke and De Meester 1997;but see Giessler 1997). Gene flow estimates derived from gene frequency divergence in invertebrate pond taxa are often low, even though the presence of resistant resting eggs and the rapid colonisation of new habitats suggest high dispersal capacities (De Meester 1996).…”

“…Studies on species from very short-lived desert pools may be particularly informative, as the impacts of chance processes such as dispersal, colonisation and genetic drift are expected to be higher in such habitats than in the lakes and ponds which are traditionally studied. The limited available information on the genetic structure of freshwater anostracans generally reveals significant levels of local differentiation (Boileau et al 1992;Riddoch et al 1994;Davies et al 1997;Bohonak 1998). This pattern was interpreted by Bohonak (1998) as a balance between drift and gene flow, while Boileau et al (1992) states that it is all historical artifact.…”