Alpha-taxonomy and genetic information on closely related estuarine copepods

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  • 汽水性カイアシ類近縁種のα分類学と遺伝子情報
  • キスイセイ カイアシルイ キンエンシュ ノ aブンルイガク ト イデンシ ジョウホウ

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<p>Genetic information (GI) has been commonly used to confirm divergence between closely related species in α-taxonomy. However, there are some difficulties in the usage of GI, especially when a new species that has been formerly identified as a different species is described. We present three examples of our recent α-taxonomic studies on estuarine copepods, in which GI was used to evaluate divergence, and discuss the advantage and difficulties of applying GI in α-taxonomy. The first example is a description of the new species Pseudodiaptomus nansei, which was formerly reported as P. inopinus. Although the longest caudal seta differs between them (generally swollen in P. inopinus, but thin in P. nansei), the difference had been considered as intra-specific variation. However, we concluded that P. nansei is a distinct species by clear differences in mitochondrial DNA (mtCOI) and other morphological characters. In this example, the GI was critical for the conclusions drawn and led us to conduct a detailed morphological comparison. The second example is for two forms of P. inopinus. Our intensive faunal surveys for estuarine copepods in western Japan revealed two morphological forms of P. inopinus, for which the distributions are strictly separated into the Japan Sea coast and the Pacific-East China Sea coasts, with a boundary at northwest Kyushu. We concluded that the two forms are distinct species by morphological variations and our unpublished GI. The Japan Sea form should be identifiable to P. japonicus Kikuchi, 1928, which was described from a brackish lake on the Japan Sea coast and has been regarded as a junior synonym of P. inopinus. However, some doubt remains that the other form is really P. inopinus s. str., because there are minor morphological differences between our specimens and the original illustrations of P. inopinus. The third example is two different size forms of Oithona dissimilis in the Nansei Islands. Both mitochondrial and nuclear DNAs showed distinct divergences between the forms, indicating that they are different species. The small and large forms are similar to the two subspecies from India, O. d. dissimilis and O. d. oceanica, respectively, in regard to their habitat conditions and body size. However, it is doubtful that the two forms in the Nansei Islands are identical to these subspecies, because of minor morphological differences and a far-distant geological distribution between them. In the latter two examples, the species names of our specimens could not be completely determined, because of the difficulty in obtaining the type specimens to be compared with our specimens, especially regarding genetic analyses. Therefore, if genetic comparisons with the type material are necessary for the creation of a new species, α-taxonomy would be more difficult to practice. In previous studies, mitochondrial GI of estuarine copepods sometimes showed large intra-specific variations. If it is unsure whether the mitochondrial GI indicates intra- or inter-specific variations, a genealogical concordance approach using both mitochondrial and nuclear GI is recommended.</p>

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