Demographic history, geographical distribution and reproductive isolation of distinct lineages of blue rockfish (Sebastes mystinus), a marine fish with a high dispersal potential

Abstract

Understanding the barriers to genetic exchange in taxonomic groups that have a high dispersal potential will provide critical information on speciation in general. Blue rockfish (Sebastes mystinus) are good taxa to examine speciation because they are nonmigratory inhabitants of shallow rocky reef habitats along the eastern North Pacific with a pelagic larval stage lasting 3–5 months. The goal of this study was to analyse the evolutionary history and distribution patterns of different lineages within S. mystinus described previously and use this information to understand the speciation process in this group of high dispersal fish. The molecular data derived from specimens sampled over approximately 1650 km of the S. mystinus range revealed a northerly and southerly distribution for the two lineages. Almost equal frequencies of both lineages occurred at centrally located sample locations, with evidence of reproductive isolation between the lineages. A demographic analysis showed that the two lineages diverged and experienced sudden expansion prior to the last glacial maximum, which affected the observed pattern of genetic structure. The spatial distribution, demographic history and degree of genetic distinctiveness found from the genetic analysis, despite the high potential for dispersal in S. mystinus, suggest both lineages diverged in allopatry.

Speciation along a depth gradient in a marine adaptive radiation

“…This finding supports the hypothesis that rockfishes speciate along a depth gradient [18], and argues against several alternative models of speciation. Allopatric speciation between northern and southern populations has been inferred for a possible incipient species pair within S. mystinus [19], but the weak signal of speciational change in latitudinal distribution suggest that fully allopatric speciation is rare in northeast Pacific Sebastes. It has also been proposed that rockfish speciation involves divergence in life history, particularly in maximum lifespan, which varies from 10 to 200 years among species [20].…”

Age-structured genetic analysis reveals temporal and geographic variation within and between two cryptic rockfish species

“…In an analysis of hybridization of settling S. mystinus juveniles, there was little or no evidence of first generation hybrids, suggesting that reproductive isolation between the 2 species in regions where both coexist was due to pre-zygotic barriers (Burford et al 2011). Within the adult populations of each species there was evidence of genetic differentiation only in the southerly-distributed species at 2 locations (Santa Rosa Island and Gaviota, both south of Point Conception), suggesting that while the long pelagic phase contributed to connectivity in the northerly-distributed species, the same was not true for the southerlydistributed species (Burford 2009). …”

“…We compared 3 consecutive year-classes, 2 year-classes throughout the sampling region (2001 and 2002) and a limited sample of the 2000 year-class from a previous study (Burford & Larson 2007), and we compared these individual year-classes to re gional and local adults from a previous study (Burford 2009) (Appendix 1). Given that most Sebastes mystinus become reproductive around the age of 5 yr, these adults were comprised of older year-classes than the juveniles sampled here, as most of the collections of adults occurred at the same time or shortly after the collections of juveniles (e.g.…”

“…However, there is evidence of recent cryptic speciation within S. mystinus (mean ± SD: 520 ± 115 kyr; Burford 2009) across its geographic range (southern California to northern Washington). The 2 species were reproductively isolated and have a north-south distribution pattern, with a northerly and southerly distributed Type 1 and Type 2 species, respectively (Burford & Bernardi 2008, Burford 2009, and a range of overlap from central/ southern Oregon to northern California (~450 km; Burford 2009). In an analysis of hybridization of settling S. mystinus juveniles, there was little or no evidence of first generation hybrids, suggesting that reproductive isolation between the 2 species in regions where both coexist was due to pre-zygotic barriers (Burford et al 2011).…”

“…Therefore large-scale genetic analyses of species within this group provide an opportunity to investigate the genetic consequences of dispersal for species with multiple year-classes and, potentially, different levels of connectivity. For this reason and because of their great economic and ecological importance along the west coast of North America, the genus Sebastes has received much attention, with development of genetic markers and previous descriptions of the spatial genetic structure of several species (Buonaccorsi et al 2002, Miller et al 2005, Burford & Bern ardi 2008, Hyde et al 2008, Burford 2009, Sivasundar & Palumbi 2010, Hess et al 2011. Due to the longevity of some species in this group, rockfishes provide an opportunity to examine how genetic variation among year-classes and the different environmental conditions they experience contribute to the genetic variation of local and regional adult populations.…”