- 魚類学雑誌 (ISSN:00215090)
- vol.47, no.1, pp.1-28, 2000-05-25 (Released:2010-06-28)
Because lampreys have been considered to be representative of primitive vertebrates, frequent taxonomic, phylogenetic and speciation studies have been made, both before and since the landmark publication, “The Biology of Lampreys” (-1971-81). Primarily because of their low diversity of morphological features (treated as valid taxonomic characteristics), lampreys have been subject to much taxonomic uncertainty. Nevertheless, current phylogenetic hypotheses, based pri-marily on dentition, have been accepted by many researchers, with only slight modifications, over the last 2-3 decades. However, recent molecular studies of some lamprey groups have demonstrated the potential for a molecular approach to phylogenetic systematics of lampreys and a new basis for evaluation of previously-held hypotheses.Many lamprey genera are composed of several species characterized by different life-styles, such as parasitic, anadromous and nonparasitic, fluvial. The speciation process proposed in previous studies has been broadly divided into the following patterns: nonparasitic, fluvial species have evolved directly from a parasitic, anadromous species; and some nonparasitic, fluvial species have evolved from an intermediate at the parasitic, fluvial stage, rather than directly from a parasitic, anadromous form. In this review, these two processes and the mechanisms by which nonparasitic forms may have evolved, are discussed.As a case study, phylogeny and speciation within the Far East monophyletic genus Lethenteron is considered. Four Lethenteron taxa, L. japonicum, L. kessleri and the northern and southern forms of L. reissneri, should be regarded as discrete species because of the existence of reproductive isolation between all possible pairs of taxa in region of sympatry. In the monophyletic group, comprising the former three species, the nonparasitic fluvial L. kessleri and the northern form of L. reissneri are both thought to have evolved from ancestral stocks of parasitic, anadromous L. japonicum, following the occurrence of precocious dwarf individuals in each an-cestral line.