Introgression between ecologically distinct species following increased salinity in the Colorado Delta- Worldwide implications for impacted estuary diversity
- Published
- Accepted
- Subject Areas
- Biogeography, Conservation Biology, Evolutionary Studies
- Keywords
- introgression, hybridization, Colorado River Delta, Gulf of California, Colpichthys, endemic species, ecological species, extinction through hybridization
- Copyright
- © 2017 Lau et al.
- Licence
- This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
- Cite this article
- 2017. Introgression between ecologically distinct species following increased salinity in the Colorado Delta- Worldwide implications for impacted estuary diversity. PeerJ Preprints 5:e3281v1 https://doi.org/10.7287/peerj.preprints.3281v1
Abstract
We investigate hybridization and introgression between ecologically distinct sister species of silverside fish in the Gulf of California through combined analysis of morphological, sequence, and genotypic data. Water diversions in the past century turned the Colorado River Delta from a normal estuary to a hypersaline inverse estuary, raising concerns for the local fauna, much of which is endangered. Salinity differences are known to generate ecological species pairs and we anticipated that loss of the fresher-water historic salinity regime could alter the adaptive factors maintaining distinction between the broadly distributed Gulf-endemic Colpichthys regis and the narrowly restricted Delta-endemic Colpichthis hubbsi, the species that experienced dramatic environmental change. In this altered environmental context, these long-isolated species (as revealed by Cytochrome b sequences) show genotypic (RAG1, microsatellites) evidence of active hybridization where the species ranges abut, as well as directional introgression from C. regis into the range center of C. hubbsi. Bayesian group assignment (STRUCTURE) on six microsatellite loci and multivariate analyses (DAPC) on both microsatellites and phenotypic data further support substantial recent admixture between the sister species. Although we find no evidence for recent population decline in C. hubbsi based on mitochondrial sequence, introgression may be placing an ancient ecological species at risk of extinction. Such introgressive extinction risk should also pertain to a number of other ecological species historically sustained by the now changing Delta environment. More broadly, salinity gradient associated ecological speciation is evident in silverside species pairs in many estuarine systems around the world. Ecological species pairs among other taxa in such systems are likely poorly understood or cryptic. As water extraction accelerates in river systems worldwide, salinity gradients will necessarily be altered, impacting many more estuary and delta systems. Such alteration of habitats will place biodiversity at risk not only from direct effects of habitat destruction, but also from the potential for the breakdown of ecological species. Thus, evolutionary response to the anthropogenic alteration of salinity gradients in estuaries merits investigation as the number of impacted systems increases around the globe, permitting parallel study of multiple systems, while also permitting a conservation management response to help preserve this little championed component of biodiversity.
Author Comment
This is a submission to PeerJ for review.
Supplemental Information
Radiographs of vertebral differences
Fig. S1 Radiographs of a C. hubbsi specimen (A) and a C. regis specimen (B); arrow in (B) shows the expanded process in the hemal spines of C. regis.
STRUCTURE assignment of individuals to groups (K=3 and K=4)
Fig. S2 Structure plots for the correlated allele frequency model for K=3 (top) and K=4 (bottom).
Boxplots of the DAPC coordinates for specimens in the hybrid zone
Fig. S3 Boxplots of the DAPC coordinates for specimens in the hybrid zone
External morphological measurements of a historic collection of Colpichthys
Table S1 Measurements of the external morphology of C. regis specimens collected in 1968 at the San Felipe site. SL = standard length; Snout-D1 = length from snout to first dorsal fin origin; DLS = dorsolateral scale count. Measurements for specimen 16 (in bold) identify it as C. hubbsi sensu Crabtree (1989). Specimens on loan from the Scripps Institute of Oceanography (SIO) collection.
Microsatellite allele frequencies
Table S2 Frequencies of each microsatellite alleles at each sampling locale. Microsatellite alleles are arranged by size. Number in parentheses under each locale denotes number of individuals analyzed.
Description of raw data formats
Description of raw data formats
C. regis Cyt b sequence data
C. regis Cyt b sequence data
C. hubbsi Cyt b sequence data
C. hubbsi Cyt b sequence data