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Ice Age processes shape present-day patterns of kelp biodiversity

 

About the author: Sarah Shainker (@SarahShainker) completed a B.S. in Marine Biology at the College of Charleston before serving as a Peace Corps volunteer in the Philippines, where she developed interests in environmental education and science communication. Sarah is a PhD student in Dr. Stacy Krueger Hadfield’s lab at the University of Alabama at Birmingham. She plans to integrate population genetics and a citizen science monitoring network to develop eco-evolutionary studies of macroalgae in the southeast United States.

 

 

During the Pleistocene Epoch, the most recent Ice Age, areas of present-day North America and Europe were covered by glaciers. Contemporary patterns of genetic diversity suggest that terrestrial organisms were pushed from these ice-covered areas to ice-less refugia, where biodiversity flourished. As a result, modern terrestrial organisms tend to show more diversity at lower latitudes. The effects of Pleistocene-age glaciers on coastal organisms, however, are less well-studied.

Fig. 1 Alaria esculenta

Winged kelps in the genus Alaria have inhabited the coastlines of the Northern Hemisphere since the Pleistocene (Fig. 1). Eight to nine species of the genus are recognized globally; however, the taxonomic diversity in the northeast Pacific is still being resolved. Grant and Bringloe (2020) aimed to assess Alaskan Alaria populations to investigate the influence of the Pleistocene on present patterns of diversity.

Alaria populations were sampled along 2800 km of coastline in the Gulf of Alaska, a larger spatial scale compared to previous studies. The authors used several types of genetic markers: microsatellites, the plastid rbcL gene, and the mitochondrial cox1 gene. Uniparentally inherited plastid and mitochondrial DNA are informative for historical relationships, whereas microsatellites are inherited biparentally and can provide information on hybridization and mutation rates, and are informative of both historical and contemporary relationships among populations. Together, these three markers create a finer-scale portrait of genetic diversity patterns than any of them could alone.

Fig. 2 Haplotype networks for rbcL (a,b), cox1 (c,d) and combined rbcL & cox1 (e,f.). Fig. 1 in Grant & Bringloe (2020)

Based on cox1 and rbcL, five genetic lineages were identified. Some lineages were highly structured at scales as small as 10 km, while others were widespread over hundreds of kilometers (Fig. 2). The small-scale structure is likely a relic of limited dispersal of propagules, and is suggestive of local adaptation. On the other hand, the unpredictable distribution of lineages over larger spatial scales may indicate population turnover due to the cycling of multiple glacial advances and retreats, resulting in different patterns of isolation.

Divergence among microsatellite genotypes, together with the haplotype networks, suggested that hybridization is occurring between different genetic lineages. These hybridization patterns suggest that divergences between these lineages are recent, at least on the scale of geological time.

The genetic groups identified with organellular DNA did not correspond with previous species names that were given based on morphology. Despite the hybridization indicated by the microsatellites, these lineages provide a good starting point for future phylogenetic work.

Unlike terrestrial species in the northeast Pacific, Alaria did not follow the expected pattern of decreasing genetic diversity at higher latitudes. Genetic diversity remained high, suggesting that there were refugia at higher latitudes with conditions suitable for Alaria’s growth and reproduction.

The use of several genetic markers over a large spatial scale provides a clearer understanding of Alaria’s patterns of genetic diversity, and hints at the historical mechanisms driving those patterns.

 

Reference:

Grant, W. S. & Bringloe, T. T. (2020). Pleistocene ice ages created new evolutionary lineages, but limited speciation in the Northeast Pacific winged kelp. Journal of Heredity, esaa053, https://doi.org/10.1093/jhered/esaa053.

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