Molecular phylogenetic relationship between Trichiurus margarites Li, 1992 and Trichiurus sp.2 (sensu Nakabo, 2002) (Perciformes: Trichi-uridae) based on mitochondrial 16S rRNA sequence analysis

Abstract

The taxonomic identification of the genus Trichiurus(Trichiuridae) has historically been challenging because of the morphological similarities between the species.As a result,Trichiurus lepturus Linnaeus,1758 was recognized as a monotypic cosmopolitan species of Trichiurus prior to 1990.In 1992,Li proposed the new species T.margarites Li,1992,based on morphological differences in the skull patterns and dorsal fin coloration,i.e.su-praoccipital bone with a hard knob and yellow dorsal fin respectively.Subsequently,Yamada et al.described a species of Trichiurus with a yellow dorsal fin from the Ryukyu Islands and East China Sea coast of Kyushu,Japan.This species is very similar to T.margarites,but shows some variation in the size of a pair of knobs on the su-praoccipital crest.Accordingly,the yellow dorsal fin species of Trichiurus from Japan was provisionally called Trichiurus sp.2 by Nakabo.There remains no molecular evidence that elucidates the genetic differences between T.margarites and T.sp.2(sensu Nakabo,2002).Thus,the phylogenetic relationship and taxonomic status of these two species remain unresolved. In this study we examined the mitochondrial 16S rRNA gene sequence from 11 T.margarites individuals collected from the coast of western Hainan Island(Baimajing,Yinggehai and Sanya),South China Sea,China which is the type locality of T.margarites.The 16S rRNA genes were amplified using PCR techniques.Homolo-gous sequences from T.sp.2 and closely-related species from previous studies were also included in the compara-tive analysis.Genetic information indexes,including base composition,sequence variation,and Kimura-2 pa-rameter net genetic distance were examined.Phylogenetic analysis using neighbor-joining(NJ) and maxi-mum-likelihood(ML) methods were conducted using Eupleurogrammus muticus as the outgroup. There was evidence of insertion or deletion of base pairs in the 435 bp sequence alignment,with 63 variable sites,48 parsimony informative sites,and 15 singleton sites.The net genetic distances between T.margarites(from western Hainan Island) and T.sp.2(from Japan,eastern Hainan Island,and Indian Ocean) ranged from 0.000 to 0.006.These values were much lower than those found for interspecific 16S rRNA comparisons in some species of Trichiurus(0.037–0.061),and for intraspecies comparisons between populations of T.lepturus(0.010–0.016).Moreover,the haplotypes of T.margarites and T.sp.2 did not form reciprocal monophyletic clades in the phylogenetic trees(NJ,ML),and the close relationship of the haplotypes of these two species was supported by high confidence levels(NJ 100%,ML 99%).These results indicated that the genetic distance between T.mar-garites and T.sp.2 was at the intraspecific level,and the species T.sp.2(sensu Nakabo,2002) from Japan should be classified as T.margarites Li,1992.It has been suggested that T.margarites is widely distributed from the In-dian Ocean to the western Pacific,and not merely limited to the South China Sea,Taiwan Strait,and Taiwan coastal waters.Based on coalescent estimates of divergence times among species,the origin and evolution of T.margarites are primarily discussed in this study.