Hidden Floridian biodiversity: mitochondrial and nuclear gene trees reveal four cryptic species within the scorched mussel,Brachidontes exustus, species complex

Abstract

The well-documented Floridian 'Gulf/Atlantic' marine genetic disjunction provides an influential example of vicariant cladogenesis along a continental coastline for major elements of a diverse nearshore fauna. We are engaged in a two-part study that aims to place this disjunction into a regional Caribbean Basin phylogenetic perspective using the scorched mussel Brachidontes exustus as an exemplar. Our first step, documented here, is to thoroughly characterize the genetic structure of Floridian scorched mussel populations using mitochondrial (mt) and nuclear markers. Both sets of markers recovered the expected disjunction involving sister clades distributed on alternate flanks of peninsular Florida and lineage-specific mt molecular clocks placed its origin in the Pliocene. The two sister clades had distinct population genetic profiles and the Atlantic clade appears to have experienced an evolutionarily recent bottleneck, although plots of the relative estimates of N through time are consistent with its local persistence through the last Ice Age Maximum. Our primary novel result, however, was the discovery that the Gulf/Atlantic disjunction represents but one of three cryptic, nested genetic discontinuities represented in Floridian scorched mussel populations. The most pronounced phylogenetic split distinguished the Gulf and Atlantic sister clades from two additional nested cryptic sister clades present in samples taken from the southern Florida tropical marine zone. Floridian populations of B. exustus are composed of four cryptic taxa, a result consistent with the hypothesis that the Gulf/Atlantic disjunction in this morphospecies is but one of multiple latent regional genetic breakpoints.

Placing the Floridian Marine Genetic Disjunction Into a Regional Evolutionary Context Using the Scorched Mussel, Brachidontes Exustus, Species Complex

“…3, geminate 2). The remaining Bahamas and Antilles (i.e., Key Biscayne clade of Lee and Ó Foighil, 2004) nominal B. exustus sister clades lacked putative geminates but the former surprisingly incorporated Cape Verde B. puniceus nominal congeners, most clearly for the nuclear rDNA markers (Fig. 3, indicated by asterisks).…”

“…Total genomic DNA was isolated from the posterior adductor muscle to avoid male gonadal tissue enriched with paternally transmitted ''male'' mitochondrial genomes prevalent in Mytilidae (Rawson and Hilbish 1995;Quesada et al 1996;Lee and Ó Foighil 2004). We are confident that this measure was successful due to the lack of heteroplasmy observed in our direct sequences and to the topological congruence among our mitochondrial and nuclear markers .…”

“…The previous Floridian study ( Lee and Ó Foighil 2004) recovered a Gulf/Atlantic genetic disjunction in which reciprocally monophyletic sister groups, the Gulf and Atlantic clades, were found on the respective flanks of the Floridian In the parsimony network, the observed COI haplotypes are represented by ovals sized according to their relative abundance (numbers Ͼ1 given in parentheses) and labeled and color-coded to reflect sampling location. Smaller unlabeled circles represent inferred missing haplotypes and genotypes for B. puniceus from Cape Verde are marked by asterisks.…”

“…All novel DNA sequences have been deposited in GenBank (COI: AY825105-AY825222, 28S: AY825079-AY825104, ITS1: AY825223-AY625245). Previously generated sequences (AY621835-AY622009, Lee and Ó Foighil 2004) representing Floridian genetic variation were also included and a few sequences, mainly for outgoup taxa, were retrieved from the GenBank database (see Table 1). …”

Characteristics of mitochondrial DNA of unionid bivalves (Mollusca: Bivalvia: Unionidae). I. Detection and characteristics of doubly uniparental inheritance (DUI) of unionid mitochondrial DNA

“…Most researchers of freshwater bivalves use somatic tissues as the source of DNA, and thus obtain exclusively F haplotype of mtDNA, ignoring the possibility of identifying M haplotype and detecting DUI (STÊ-PIEÑ et al 1999, GRAF & Ó FOIGHIL 2000a, GIRIBET & WHEELER 2002, LEE & Ó FOIGHIL 2004, ARAUJO et al 2005, KÄLLERSJO et al 2005, SOROKA 2005. In order to obtain M type in Unionidae, DNA should be isolated from male gonads (HOEH et al 2002b, MOCK et al 2004, WALKER et al 2006a, b, SOROKA 2008a.…”