Abstract
The genus Corbicula consists of estuarine or freshwater clams native to temperate/tropical regions of Asia, Africa, and Australia that collectively encompass both sexual species and clonal (androgenetic) lineages. The latter have become globally invasive in freshwater systems and they represent some of the most successful aquatic invasive lineages. Previous studies have documented four invasive clonal lineages, Forms A, B, C, and Rlc, with varying known distributions. Form A (R in Europe) occurs globally, Form B is found solely in North America, mainly the western United States, Form C (S in Europe) occurs both in European watersheds and in South America, and Rlc is known from Europe. A putative fifth invasive morph, Form D, was recently described in the New World from the Illinois River (Great Lakes watershed), where it occurs in sympatry with Forms A and B. An initial study showed Form D to be conchologically distinct: possessing rust-colored rays and white nacre with purple teeth. However, its genetic distinctiveness using standard molecular markers (mitochondrial cytochrome c oxidase subunit I and nuclear ribosomal 28S RNA) was ambiguous. To resolve this issue, we performed a phylogenomic analysis using 1,699–30,027 nuclear genomic loci collected via the next generation double digested restriction-site associated DNA sequencing method. Our results confirmed Form D to be a distinct invasive New World lineage with a population genomic profile consistent with clonality. A majority (7/9) of the phylogenomic analyses recovered the four New World invasive Corbicula lineages (Forms A, B, C, and D) as members of a clonal clade, sister to the non-clonal Lake Biwa (Japan) endemic, Corbicula sandai. The age of the clonal clade was estimated at 1.49 million years (my; ± 0.401–2.955 my) whereas the estimated ages of the four invasive lineage crown clades ranged from 0.27 to 0.44 my. We recovered very little evidence of nuclear genomic admixture among the four invasive lineages in our study populations. In contrast, 2/6 C. sandai individuals displayed partial nuclear genomic Structure assignments with multiple invasive clonal lineages. These results provide new insights into the origin and maintenance of clonality in this complex system.
Highlights
The clam genus Corbicula contains both sexual and asexual forms, with the former restricted to fresh or brackish waters in their native range in the temperate/ tropical regions of Asia, Africa, and Australia (Glaubrecht & Korniushin, 2003; Morton, 1986; Pigneur et al, 2014; Sousa, Antunes & Guilhermino, 2008)
Using the two clustering approaches outlined in Haponski, Lee & Ó Foighil (2019), we examined the invasive Corbicula lineages and C. sandai for further population sub-division with the Bayesian based Structure v2.3.4 (Hubisz et al, 2009; Pritchard, Stephens & Donnelly, 2000) and discriminant analysis of principal components (DAPC; Jombart, Devillard & Balloux, 2010) analyses
The numbers of inferred homologous loci for the four invasive Corbicula forms were similar across samples, the different similarity thresholds (85%, 90%, and 95%) and taxon coverages (75%, 50%, and 25%; Table S3) with the exception of one Form C individual (COR3–6) that had by far the lowest number of identified loci (372–1,297) compared to the other invasive clams (1,116–24,443)
Summary
The clam genus Corbicula contains both sexual and asexual forms (see Lee et al, 2005 and references therein; Hedtke, Glaubrecht & Hillis, 2011; Pigneur et al, 2011, 2012, 2014), with the former restricted to fresh or brackish waters in their native range in the temperate/ tropical regions of Asia, Africa, and Australia (Glaubrecht & Korniushin, 2003; Morton, 1986; Pigneur et al, 2014; Sousa, Antunes & Guilhermino, 2008). Asexual Corbicula lineages have been invading freshwater ecosystems across the globe for almost a century and are considered a major aquatic pest (summarized by Tiemann et al, 2017) Their first documented appearance outside of their native range occurred in western North America in the 1920s (summarized by Lee et al, 2005; Tiemann et al, 2017) and since they have reportedly spread throughout North and much of South America (Beasley, Tagliaro & Figueiredo, 2003; Lee et al, 2005) and across Europe (Pigneur et al, 2011, 2014; Crespo et al, 2015; Peñarrubia et al, 2017), with Antarctica being the only continent with no known occurrences (Crespo et al, 2015; GBIF.org, 2019). A fourth invasive European lineage (Rlc) has yet to be reported in the New World (Pigneur et al, 2014)
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