Molecular phylogeny of the sacoglossa, with a discussion of gain and loss of kleptoplasty in the evolution of the group.

Sacoglossan sea slugs retain the chloroplasts from food algae in their cells (kleptoplasty) and obtain the photosynthetic products, but the capability of kleptoplasty differs among species. One evolutionary hypothesis for kleptoplasty is that the family Volvatellidae is the ancestral sacoglossan lineage in which kleptoplasty arose, but this is based on ambiguous phylogenetic relationships. To test this hypothesis, we analyzed sacoglossan phylogeny for concatenated sequences (approximately 4000 bp) of nuclear (18S rRNA) and mitochondrial (CO1, trnV, and 16S rRNA) genes of 18 sacoglossan species, using neighbor-joining, maximum parsimony, maximum likelihood, and Bayesian methods. The resultant phylogenetic trees clearly revealed monophyly of the order Sacoglossa (including the debatable genus Cylindrobulla), which contained two sister clades: the shelled sacoglossan group (Oxynoacea + Cylindrobulla) and the non-shelled group (Plakobranchacea). The family Volvatellidae was in a derived position within the Oxynoacea. The most parsimonious rearrangement of the character states of kleptoplasty on the phylogenetic tree suggested that (1) kleptoplasty was acquired at the basal position of the Sacoglossa (including Cylindrobulla) as non-functional kleptoplasty, (2) functional kleptoplasty was gained in the Plakobranchacea clade, and (3) non-functional kleptoplasty was lost in the Volvatellidae lineage in the Oxynoacea clade.