Sympatric size variants of the microcopepod Oncaea venusta exhibit distinct lineages in DNA sequences

We investigated the phylogenetic relationships among different size groups of the pelagic microcopepod Oncaea venusta Philippi, 1843, by comparing the patterns of genetic variation of specimens collected at five locations of the Indo-West Pacific Ocean. Phylogenetic analyses were based on sequence data obtained from two DNA markers: A 310 bp fragment of the mitochondrial cytochrome b (cyt b) gene and a 480 bp fragment of the nuclear internal transcribed spacer 1 (ITS1). The cyt b sequences showed a much higher level of variation than those from ITS1, but the conclusion from both genes was concordant. Four genetic clades could be differentiated. A small- and a large-size group were unambiguously assigned to two distinct clades or lineages. Unexpectedly, the medium-sized individuals could be divided into another two different genetic clades. All four lineages were supported by high bootstrap values. The high levels of sequence divergence under sympatric conditions indicated that at least the two main groups, the large and the small one, may be assigned to different species. For the medium-size group additional morphological studies and more sensitive nuclear markers are required to clarify their taxonomic status.