Abstract
BackgroundThe intertidal copepod Tigriopus californicus is a model for studying the process of genetic divergence in allopatry and for probing the nature of genetic changes that lead to reproductive isolation. Although previous studies have revealed a pattern of remarkably high levels of genetic divergence between the populations of this species at several spatial scales, it is not clear what types of historical processes are responsible. Particularly lacking are data that can yield insights into population history from the finest scales of geographic resolution.ResultsSequence variation in both cytochrome b (CYTB, mtDNA) and the rieske iron-sulfur protein (RISP, nuclear) are examined at a fine scale within four different regions for populations of T. californicus. High levels of genetic divergence are seen for both genes at the broader scale, and genetic subdivision is apparent at nearly all scales in these populations for these two genes. Patterns of polymorphism and divergence in both CYTB and RISP suggest that selection may be leading to non-neutral evolution of these genes in several cases but a pervasive pattern of neither selection nor coadaptation is seen for these markers.ConclusionThe use of sequence data at a fine-scale of resolution in this species has provided novel insights into the processes that have resulted in the accumulation of genetic divergence among populations. This divergence is likely to result from an interplay between a limited dispersal ability for this copepod and the temporal instability of copepod habitat. Both shorter-term processes such as the extinction/recolonization dynamics of copepod pools and longer-term processes such as geological uplift of coastline and sea level changes appear to have impacted the patterns of differentiation. Some patterns of sequence variation are consistent with selection acting upon the loci used in this study; however, it appears that most phylogeographic patterns are the result of history and not selection on these genes in this species.
Highlights
The intertidal copepod Tigriopus californicus is a model for studying the process of genetic divergence in allopatry and for probing the nature of genetic changes that lead to reproductive isolation
Population sampling To investigate genetic divergence in mtDNA and population history among closely spaced Tigriopus californicus populations, copepods were collected from high intertidal rock pools spaced along 3–16 km stretches of coastline centered at four rocky outcrops in California that have been used extensively in previous studies of T. californicus: San Diego (SD) on Point Loma and La Jolla Point (LJP) both in San Diego County, Abalone Cove (AB) on the Palos Verdes Peninsula in Los Angeles County, and Santa Cruz (SCN) in central California
Defining cryptic species in allopatric populations is a difficult problem, a number of factors suggest that T. californicus may be still considered a single biological species: These populations are able to interbreed in the lab and produce hybrids, including advanced generation hybrid lineages [24,44], there is no evidence for premating isolation in crosses between populations [45,46], and little consistent morphological divergence among populations has been found in studies done to date [12,47]
Summary
The intertidal copepod Tigriopus californicus is a model for studying the process of genetic divergence in allopatry and for probing the nature of genetic changes that lead to reproductive isolation. Over time local population extinctions are likely, due to the ephemeral nature of copepod habitat [14,15] and long distance gene flow must occur occasionally as evidenced by the recolonization of previously glaciated regions [5] Most of this previous work primarily utilized allozyme markers at the finest scales of geographic resolution with the result that patterns of DNA sequence variation have not yet been studied at the geographic scale necessary to yield insights into the longerterm historical processes of extinction and recolonization
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