Spatial genetic features of eastern oysters (Crassostrea virginica Gmelin) in the Gulf of Mexico: northward movement of a secondary contact zone

Abstract

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (FST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long-term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23-year span.

Population Structure of Atlantic Croakers from the Gulf of Mexico: Evaluating a Single‐Stock Hypothesis Using a Genomic Approach

“…A final interesting observation was the significant mtDNA divergence measured between two western Gulf samples: Laguna Madre, Texas, and Cedar Lakes, Texas. Localized divergence in Laguna Madre versus other sites in the Gulf has been observed previously in other species and could be the result of either a secondary contact zone between historically isolated populations (as in eastern oysters Crassostrea virginica; King et al 1994;Anderson et al 2014) or isolation by distance acting on overlapping subpopulations (as in other sciaenid finfishes; Gold et al 1999aGold et al , 1999bGold and Turner 2002;Anderson and Karel 2009). Absence of significant divergence of the Laguna Madre sample with neutral SNP data (described below) supports the former, suggesting that previously isolated populations of Atlantic Croakers are now experiencing secondary contact with one another between Laguna ATLANTIC CROAKER POPULATION STRUCTURE Madre and areas further north.…”

“…; Anderson et al. ) or isolation by distance acting on overlapping subpopulations (as in other sciaenid finfishes; Gold et al. , 1999b; Gold and Turner ; Anderson and Karel ).…”

Morphological Assessment of the Eastern Oyster Crassostrea virginica throughout the Gulf of Mexico

“…(Dahlberg 1970;McClure and McEachran 1992;Portnoy and Gold 2012) and indicates that Apalachicola may indeed represent a taxonomic boundary between divergent oyster populations, harboring both general types of shell morphology (northern and eastern). In Texas, previous work has elicited significant genetic structure centered around Aransas Bay and a well-defined transition zone between divergent "northern" and "western" Texas populations based on studies with a gulfwide focus (Hoover and Gaffney 2005;Varney et al 2009) as well as a Texas-specific focus (King et al 1994;Anderson et al 2014). Genetic divergence between eastern oysters from the Lower Laguna Madre (Texas) and elsewhere in Texas was first observed by Groue and Lester (1982), who also reported that Laguna Madre oysters were significantly smaller and lighter than oysters from other Gulf of Mexico locations.…”

“…For sampling inside of Texas, eastern oysters were collected by hand or dredged from known oyster reefs during routine monthly oyster reef monitoring by Texas Parks Wildlife Department. A total of 30 eastern oysters were randomly collected from each bay, in each state, except for the Mission-Aransas Bay system in Texas, an area where two highly divergent genetic populations of eastern oysters coincide (Anderson et al 2014). The Mission-Aransas estuary is composed of Mesquite, Copano, Aransas, and Redfish bays; 15 eastern oysters were collected from each of these areas.…”

Assessing the genetic diversity of the mangrove oysterCrassostrea rhizophorae(Bivalvia, Ostreidae) by microsatellite markers in southeastern Brazil

“…Microsatellite markers have also been used successfully in many countries to assess genetic variation of oysters from both wild and farm stocks (Yu et al 2008;Varney et al 2009;Xiao et al 2010Xiao et al , 2011Miller et al 2012;Meistertzheim et al 2013;Anderson et al 2014). In Brazil, previous studies have evaluated the structure and variability of native oyster stocks using other markers, such as allozymes and mitochondrial DNA (Ignacio et al 2000;Lapègue et al 2002;Melo et al 2010;Lazoski et al 2011), but no microsatellite-based assessment has been carried out with native oyster species in Brazil to date.…”