by Cameron Lukos
The genetic consequences of being at the edge of species ranges has been the subject of much debate. Populations that occur at low latitude ranges are expected to retain high unique genetic diversity. Less favorable environments that limit population size at the range edges may have caused genetic erosion that has a stronger effect than past events. This study by Assis et al. (2013) provided a test of whether the population declines at the peripheral range might be shown in decreasing diversity and increasing population isolation and differentiation. The authors compared population genetic differentiation and diversity with trends in abundance along a latitudinal gradient to the furthest extents of the range of a sea kelp, Saccorhiza polyschides. Assis et al. also looked at recent bottleneck events to determine whether the recent recoded distributional shifts had a negative impact on the population size. They found that there was decreasing population density and increasing spatial fragmentation and local extinction at the southern edge. The genetic data revealed two distinct groups and a central mixed group. As the authors had predicted there was higher differentiation and evidence of bottleneck at the southern edge but instead of a decrease there was an increase in genetic diversity suggesting that extinction and recolonization had not reduced diversity and that this may be evidence of a process of shifting genetic baselines.
Assis et al. focused on the Portuguese coastline. They divided the coastline into twenty five twenty five kilometer x twenty five kiolmeter cells. Their sampling was intensified at the three northern-most and southern-most cells, by dividing the cells into five kilometer cells. They took their samples at comparable depths during the summers of 2008 and 2010. To evaluate the variability in the distribution and abundance of S. polyschides, Assis et al. plotted the presence and absence with a list of historical geo-referenced occurrences. To test genetics, they used genomic DNA using a CTab method and filter plates. The allele sizes were scored using STRand software and put into classes using MsatAllele. Genetic diversity was determined using FSTAT and the data were placed in a linear regression model. Bottleneck effects were tested using two methods: 1. Heterozygosity excess 2. M-ratios.
Assis et al. showed that there is a persistence of high unique genetic diversity at the species range edge. They found a decrease in the density and an increase in fragmentation with latitude, but the hypothesis of a decrease in genetic diversity with decreasing density was not confirmed. Allelic richness and heterozygosity increased towards the more sparsely populated south edge and the southern sites were strongly genetically different. The results raise the question of why genetic population diversity was higher at a low latitude edge. One hypothesis was the occurrence of microscopic development stages, but this is not supported by any of their data. An alternative hypothesis is the persistence of suitable habitat refugia at the southern locations.
Assis, J., Coelho, N.C., Alberto, F., Valero, M., Raimondi, P., Reed, D., Serrão, E.A., 2013. High and distinct range-edge genetic diversity despite local bottlenecks. PloS one 8, e68646.1932-6203. Full paper at: http://bit.ly/1qKsMnV