Phylogenetic analysis of the Sphaeriidae (Mollusca: Bivalvia) based on partial mitochondrial 16S rDNA gene sequences

Abstract

We have constructed molecular phylogenetic trees for members of the Sphaeriidae in order to test proposed generic level relationships, and to reconstruct the evolutionary pattern of parental care, in this exclusively freshwater family of heterodont bivalves. An ∼480 nucleotide fragment of the mitochondrial large ribosomal subunit (16S rDNA) was sequenced for 4 corbiculid outgroups in addition to 19 sphaeriid ingroup taxa. Ingroup species were obtained from North and South America, Europe, and Australasia and included representatives of the main sphaeriid genera. Our analyses support four primary conclusions: 1) the Sphaeriinae are robustly monophyletic with respect to Eupera platensis; 2) the genus Pisidium is paraphyletic and P. sterkianum is sister to the 17 other sphaeriine taxa in our dataset; 3) synchronous brooding is the ancestral reproductive pattern in the Sphaeriinae; 4) the sequential brooders form a clade in which Musculium taxa are monophyletic and nested among lineages of Sphaerium. Our gene trees reveal an evolutionary progression in parental care complexity from the relatively simple pattern in the Euperinae, to the origin of brood sacs and of extraoogomal embryonic nutrition in the common ancestor of the Sphaeriinae, and ultimately to the development of sequential brooding in Sphaerium/Musculium taxa.