Melroy LM and Cohen CS (2021), Temporal and spatial variation in population st...

ECB-ART-49241

Ecol Evol 2021 Apr 01;117:3313-3331. doi: 10.1002/ece3.7283.

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Temporal and spatial variation in population structure among brooding sea stars in the genus Leptasterias.

Melroy LM , Cohen CS .

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Temporal genetic studies of low-dispersing organisms are rare. Marine invertebrates lacking a planktonic larval stage are expected to have lower dispersal, low gene flow, and a higher potential for local adaptation than organisms with planktonic dispersal. Leptasterias is a genus of brooding sea stars containing several cryptic species complexes. Population genetic methods were used to resolve patterns of fine-scale population structure in central California Leptasterias species using three loci from nuclear and mitochondrial genomes. Historic samples (collected between 1897 and 1998) were compared to contemporary samples (collected between 2008 and 2014) to delineate changes in species distributions in space and time. Phylogenetic analysis of contemporary samples confirmed the presence of a bay-localized clade and revealed the presence of an additional bay-localized and previously undescribed clade of Leptasterias. Analysis of contemporary and historic samples indicates two clades are experiencing a constriction in their southern range limit and suggests a decrease in clade-specific abundance at sites at which they were once prevalent. Historic sampling revealed a dramatically different distribution of diversity along the California coastline compared to contemporary sampling and illustrates the importance of temporal genetic sampling in phylogeographic studies. These samples were collected prior to significant impacts of Sea Star Wasting Disease (SSWD) and represent an in-depth analysis of genetic structure over 117 years prior to the SSWD-associated mass die-off of Leptasterias.

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	FIGURE 1. Maximum Likelihood tree for concatenated DâLoop and COI mtDNA haplotypes of Leptasterias. Circles represent nodal support: right side shading represents bootstrap values of 70 or greater and left side shading represents Bayesian Posterior Probabilities of 95% or greater. Colors indicate groupings of haplotypes into clades. Reference sequences for L.Â aequalis clades D, K, A, and B and L.Â hexactis were obtained from GenBank
	FIGURE 2. BEAST consensus tree for Leptasterias concatenated DâLoop and COI mtDNA haplotypes constructed with Bayesian MCMC analysis (Drummond etÂ al.,Â 2012). Leptasterias camtshatica was included as the outgroup sister taxon. The L.Â hexactis and L.Â aequalis split, marked with a red asterisk, was estimated by Foltz etÂ al.,Â (2008) and used for calibration. Reference sequences were included from GenBank (see text for accession numbers). Black circles represent Bayesian Posterior Probability nodal support of 95% or greater. Estimates of divergence times are shown in millions of years and numbers in parentheses are 95% highest posterior density intervals. The scale bar shows the expected number of substitutions per site and the bottom grid axis represents time in millions of years with 0.0 as present day. Colors represent clades and are consistent with colors from FigureÂ 1. Time to most recent common ancestor (TMRCA) is shown for each clade
	FIGURE 3. Clade frequencies of Leptasterias populations for (a) contemporary samples from central California to Alaska, and (b) historic samples collected in California. Colors represent clades and correspond to those found in FigureÂ 1. Letters represent sample site, see TableÂ 1, (nÂ =Â sample size), numbers above the circles represent the collection year, and circle size is representative of sample size
	FIGURE 4. Haplotype network for concatenated DâLoop and COI mtDNA haplotypes (middle) and nuclear i51 haplotypes with indels as informative characters (right). Circles represent haplotypes and circle size represents the frequency of haplotypes. Black circles represent missing haplotypes. Colors indicate population regions corresponding to the map (left). Grey shading represents clade delineation of haplotypes from phylogenetic analysis
	FIGURE 5. Mismatch distribution of pairwise distances among concatenated DâLoop and COI mtDNA haplotypes for (a) Leptasterias spp. Clade Y haplotypes (Harpening's raggedness value rÂ =Â 0, pâvalueÂ =Â 1), and (b) L.Â aequalis K haplotypes (Harpening's raggedness value rÂ =Â 0.15, pâvalueÂ =Â 0.41) compared to expected frequencies (calculated in DnaSP v5.10 and Arlequin v3.5)
	FIGURE 6. Frequency clade map for Leptasterias spp. between 1897 and 2014. Clades were delineated from COI haplotypes. Letters represent site code, size of circles represent sample size (see TableÂ 1) and colors represent clade delineation (see FigureÂ 1)

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