California Sea Grant College Program
Molecular Phylogenetics and Population Structure Derived from Mitochondrial DNA Sequence Variation in the Edible Goose Barnacle Genus Pollicipes (Cirripedia, Crustacea)
- Author(s): Van Syoc, Robert J.
- et al.
Individuals within a geographically widespread species may face considerably different environmental and ecological conditions depending on which part of the range they inhabit. This is especially true for species with long latitudinal distributions. Natural selection pressure may lead to genetic divergence and eventual speciation if the homogenizing effects of broad based gene flow are insufficient to prevent it. Therefore, clinal variation may be due either to local environmental influence on gene expression or to natural selection and reduced gene flow at the edges of the distribution.
Mitochondrial DNA sequence data and morphological characters for three species of edible goose barnacles are compared in the examination of three situations which exhibit varying degrees of potential gene flow. The first of these is possible genetic clinal variation within an apparently continuous population of Pollicipes polymerus, having a 3,300 km latitudinal distribution. The second is genetic divergence between seemingly geographically separate sub-populations of a congener, Pollicipes elegans, with a 4,400 km latitudinal distribution. The last question concerns the genetic relationships between these two eastern Pacific Ocean species, and a third, geographically isolated congener from the eastern Atlantic Ocean, Pollicipes pollicipes.
Surprisingly, the present data do not reveal a genetic discontinuity or latitudinal gradient in Pollicipes polymerus, despite the fact that the distribution of this species crosses a major marine biogeographic boundary between the Oregonian and Californian faunal provinces. This finding contradicts the hypothesis that differing reproductive types north and south of Pt. Conception are due to reduced gene flow and genetic divergence.
The disjunct (paramphitropical) sub-populations of Pollicipes elegans have a net nucleotide sequence divergence of about 1.2%. Calculated estimates for the timing of this genetic divergence range from 292,500 to 1,260,000 years before present. A divergence of this magnitude coincides with Pleistocene Epoch cooling periods and the narrowing or disappearance of the warmer sea surface temperatures forming the north equatorial barrier between present day sub-populations. This "ice age" timing for genetic interchange is not unexpected. However, the present data offer evidence for multiple transgressions of the equatorial barrier in both directions, rather than a single limited exchange.
Pollicipes, represented by three extant species, has a Tethyan distribution. Curiously, Pollicipes elegans, from the eastern Pacific, is more similar to Pollicipes pollicipes, the eastern Atlantic species, than it is to P. polymerus from the northeastern Pacific. Estimated time of genetic divergence for these two species is about 37 million years before present, near the Eocene/Oligocene boundary, when the Tethys Sea was uninterrupted and the Atlantic was significantly narrower that it is today.