A Major Mitochondrial Gene Rearrangement Among Closely Related Species

The sequential order of genes in animal mitochondrial DNA tends to be conserved at high taxonomic levels. Consequently, when mitochondrial gene rearrangements occur, they can provide compelling markers of ancient phylogenetic history (see reviews in Boore, Daehler, and Brown [1999], Saccone et al. [1999], and references therein). Such gene order characters have proven particularly useful for resolving deep-branching events characterizing superphyla, phyla, and classes, a scale over which nucleotide sequence analysis has often proven problematic (e.g., Smith et al. 1993; Boore and Brown 1994a, 1998, 2000; Boore et al. 1995; Boore, Lavrov, and Brown 1998; Stechmann and Schlegel 1999). Most frequently, these rearrangements are associated with the movement of tRNA genes which, based on their stem-and-loop structure, may also have an underlying role as mobilizing elements within the mitochondrial genome (Moritz and Brown 1987; Stanton et al. 1994; Saccone et al. 1999). Gene rearrangements involving ribosomal and protein-encoding genes (hereinafter referred to as ‘‘major’’ genes), however, are much rarer. Here we report a mitochondrial gene rearrangement involving major genes that has occurred within a genus of marine vermetid gastropods that diversified within the Cenozoic. Based on data from the fossil record and an independent molecular calibration, we estimate that this gene order change may have occurred within the last 38–48 Myr, making this a very recent major gene order rearrangement. This finding thus has important implications for the use of gene order characters in metazoan phylogeny, in terms of both understanding the mechanism of gene order rearrangements and determining the taxonomic level and timescale over which these major rearrangements may prove useful. The family Vermetidae comprises seven genera with ;100 extant species of sessile, uncoiled, suspension-feeding gastropods found in shallow marine waters in warm temperate to tropical latitudes throughout the world (Keen 1961; Morton 1965; unpublished data). Based on our current understanding of gastropod evolution, vermetid gastropods radiated from a basal caenogastropod stock sometime in the early Cenozoic (Keen 1961; unpublished data). As part of a study to investi-