Population structure and local differentiation in the delicate loach, Niwaella delicata, as revealed by mitochondrial DNA and morphological analyses

Population structures of the delicate loach, Niwaella delicata. were inferred from morphology and restriction fragment length polymorphism (RFLP) analysis of part of the mitochondrial DNA (mtDNA) of 25 populations, representing the species range in central Honshu Island. The existence of two types of morphological variation corresponding to regional distributions, the "Pacific slope type". and "Sea of Japan slope type." has been known in N. delicata. Our morphological reexamination of the two types revealed some discrepancies in their distribution pattern. Therefore, we reclassified two new color types corresponded to their distribution areas as "gathered spots type (G type)" and "scattered spots type (S type)," respectively. The: present classification of G and S types is closely related to the mtDNA divergence pattern. The current analysis also indicated that each G and S type population was further divided into two genetic groups, corresponding to geographic proximity. In spite of marked morphological differentiation, the genetic diversity between G and S type populations (1.153%) was comparable only to that reported for intraspecific levels in most freshwater fishes. Moreover, in the population of which the color patterns of all fish were characterized to the S type, mtDNA haplotypes corresponding to G and S types were sympatrically detected. This result indicates secondary contact between the two type populations and the possibility that they are not reproductively isolated.