The Quantitative and Molecular Genetic Architecture of a Subdivided Species

Abstract

In an effort to elucidate the evolutionary mechanisms that determine the genetic architecture of a species, we have analyzed 17 populations of the microcrustacean Daphnia pulex for levels of genetic variation at the level of life-history characters and molecular markers in the nuclear and mitochondrial genomes. This species is highly subdivided, with approximately 30% of the variation for nuclear molecular markers and 50% of the variation for mitochondrial markers being distributed among populations. The average level of genetic subdivision for quantitative traits is essentially the same as that for nuclear markers, which superficially suggests that the life-history characters are diverging at the neutral rate. However, the existence of a strong correlation between the levels of population subdivision and broadsense heritabilities of individual traits argues against this interpretation, suggesting instead that the among-population divergence of some quantitative traits (most notably body size) is being driven by local adaptation to different environments. The fact that the mean phenotypes of the individual populations are also strongly correlated with local levels of homozygosity indicates that variation in local inbreeding plays a role in population differentiation. Rather than being a passive consequence of local founder effects, levels of homozygosity may be selected for directly for their effects on the phenotype (adaptive inbreeding depression). There is no relationship between the levels of variation within populations for molecular markers and quantitative characters, and this is explained by the fact that the average standing genetic variation for life-history characters in this species is equivalent to only 33 generations of variation generated by mutation.

Adaptation to local ultraviolet radiation conditions among neighbouringDaphniapopulations

“…Such levels of differentiation are typical of Daphnia, even among neighbouring populations [1,18,34,35]. Among the seven ponds in our study, the inferred number of ancestral population clusters was 3 ( figure 3a).…”

“…(e) Life-table assay We assessed life-history characteristics in the absence of UVR using a standard experimental design [1,30] and used these data to calculate r, the intrinsic rate of increase. The number of replicates for each clone ranged from 1 to 4.…”

“…A central challenge in evolutionary biology concerns understanding the historical processes that generated current patterns of phenotypic differentiation within spatially subdivided populations of a single species [1,2]. Mechanisms underlying phenotypic divergence among populations fit into two broad categories: phenotypic plasticity without underlying genetic divergence between populations [3,4], or heritable genetic divergence in traits [5].…”

Incidence of diapause varies among populations of Daphnia pulicaria

“…Populations of the same Daphnia (Crustacea: Cladocera) species in particular exhibit considerable variation among lakes in many life-history traits (Lynch et al 1999) but scant attention has been paid to variation in use of dormancy. Temporary ponds obviously require diapause for persistence, but in lakes, whether or not a particular population of Daphnia remains active year-round varies considerably.…”

Genetic Diversity of Dalbergia monticola (Fabaceae) an Endangered Tree Species in the Fragmented Oriental Forest of Madagascar

“…The use of such markers to predict the diversity of adaptive genes or economic traits within populations has to be considered with caution (Lynch et al 1999). RAPD or cpSSrs are different from genes and may not be a good predictor of gene function diversity or of complex traits.…”