Estuary‐Level Genomic Variation Confirms Demographic and Life History Differences among Black Drum Populations in Texas

Abstract

The Black Drum Pogonias cromis is a year‐round resident in estuarine and nearshore habitats throughout the Gulf of Mexico and is common in Texas. Multiple studies have highlighted life history and demographic variation in this species among estuaries, and tagging studies conducted in Texas suggest limited migration between adjacent estuaries. While these findings raise the potential for genetic heterogeneity among estuaries via accumulation of genetic drift, such an effect has never been tested. We used 5,340 genomic single nucleotide polymorphisms from a reduced‐representation genomic library to assess the magnitude and geographic pattern of genetic heterogeneity in Black Drum sampled from multiple estuaries in Texas. We found significant genetic heterogeneity that could be partitioned into three regional groupings: northern (Sabine Lake to San Antonio Bay), central (upper Laguna Madre and its secondary bays) and southern (lower Laguna Madre). These regional groupings roughly coincide with significant differences in life history strategies noted in previous studies and suggest a regional stock structure that is anchored by two independently collected lines of evidence. There was also evidence for weak but significant genetic structure among adjacent secondary estuaries within the Baffin Bay complex. Finally, outliers from the genomic data set correlated weakly (but significantly) with differences in residual growth among individuals. These findings imply genetic heterogeneity of Black Drum among and within estuaries, correlation of growth phenotypes with a small partition of outlier loci, and correlation between genomic patterns and complex regional differences in life history phenotypes described previously. Stock assessments for this species should carefully consider regional stock definitions and model parameters because such inputs may vary significantly above and below the scale of estuaries.

Phylogeography, Population Structure, and Historical Demography of Black Drum in North America

“…We performed a second DAPC using geographic groupings: (1) lower Texas coast (upper and lower Laguna Madre), (2) upper Texas coast (all locations from Corpus Christi Bay north to Sabine Lake), (3) eastern Gulf of Mexico (Mobile and Apalachicola bays), and (4) Atlantic coast. These groupings were chosen based on significant genetic divergence observed in multiple studies (Leidig et al 2015; the companion paper Anderson et al 2021, https://doi.org/10.1002/nafm.10606) and thus represent the most parsimonious a priori approach to sample grouping for DAPC. As with the de novo approach, we used 85 principal components for the second DAPC.…”

“…Spatial autocorrelation demonstrated that populations became genetically differentiated at 244 km-slightly less than the distance (256 km) between the lower Laguna Madre and San Antonio Bay. Future studies examining genetic divergence between populations from northern and southern Texas could benefit from a genomic design, employing thousands of single nucleotide polymorphisms rather than the single mitochondrial gene and 15 microsatellite loci used in this study (Anderson et al 2021). This is particularly true in the context of observed life history differences in this area, which might reinforce low levels of population divergence via premating isolation mechanisms.…”