Refuting the six-genus classification of Penaeus s.l. (Dendrobranchiata, Penaeidae): a combined analysis of mitochondrial and nuclear genes

Ma, K. Y., Chan, T. -Y & Chu, K. H. (2011). Refuting the six-genus classification of Penaeus s.l. (Dendrobranchiata, Penaeidae): a combined analysis of mitochondrial and nuclear genes. - Zoologica Scripta, 40, 498-508. The taxonomic revision in 1997 of the shrimps formerly classified in Penaeus s.l. has been one of the most controversial issues on systematics of the decapods in recent years. Since Perez Farfante & Kensley (Penaeoid and Sergestoid Shrimps and Prawns of the World, 1997) split this long-accepted taxon into six genera, much debate has been devoted to their proposed new classification scheme; this has taken place because there are serious doubts whether the said scheme could reflect the evolutionary relationships among the 29 Penaeus s.l. species. Although these shrimps can be easily separated into several groups morphologically, whether these subdivisions are truly monophyletic and warrant a generic rank continues to be hotly debated among taxonomists. This study examined a total of 2425 bp sequences from three nuclear protein genes (enolase, phosphoenolpyruvate carboxykinase and sodium-potassium ATPase alpha-subunit), and the mitochondrial 16S and 12S rRNA gene of 18 Penaeus s.l. shrimps and 13 other species in the family Penaeidae. Our phylogenetic analyses strongly support the monophyly of Penaeus s.l. and, concurring with previous studies that used the mitochondrial genes alone, the paraphyly of both Penaeus s.s. (sensu Perez Farfante & Kensley, Penaeoid and Sergestoid Shrimps and Prawns of the World, 1997) and Melicertus, rendering them non-natural groupings. Our study reveals two lineages: Penaeus s.s. + Fenneropenaeus + Litopenaeus + Farfantepenaeus and Melicertus + Marsupenaeus, which exhibit genetic divergences comparable with those among other penaeid genera. However, all the morphological characters, which are emphasized by Perez Farfante and Kensley and used to separate Penaeus s.l., do not correlate with the grouping revealed by the present, perhaps decisive, phylogenetic result. Such disparity may arise from selection on the morphology of genitalia and convergent evolution. Our molecular data clearly refute the six-genus classification, and we advocate the restoration of the old Penaeus genus (=Penaeus s.l.) definition which is the only classification scheme with both the morphological and the molecular data being in agreement.