Yuji Yamazaki

Genetic structure and differentiation of four Lethenteron taxa from the Far East, deduced from allozyme analysis

Genetic structure and differentiation among four Lethenteron taxa, L. japonicum, L. kessleri, and two groups of L. reissneri, collected from Japan and the Far Eastern region of Russia, were investigated by electrophoretic analysis. Several complete-allele substitutions were found between all possible pairs of taxa in regions of sympatry, strongly suggesting the existence of reproductive isolation between them. Therefore, four Lethenteron taxa should be regarded as discrete species, respectively. In each taxon, the genetic variability within each population (H=0.062 – 0.127 for L. japonicum, H=0.062– 0.128 for L. kessleri, H=0.026 – 0.148 for the northern group of L. reissneri, and H = 0.015 – 0.102 for the southern group of L. reissneri) was considerable, suggesting large effective sizes for most populations. The sample of L. japonicum collected from Kyiya, the basin of Amur, was somewhat divergent from the other intraspecific samples. This may have resulted mainly because it does not migrate to the sea. In contrast L. kessleri has a fluvial life style, but the genetic differentiation between populations (GST=0.117, D = 0.000 – 0.061) was less than that for each group of L. reissneri (GST = 0.493, D=0.000 – 0.226 for the northern group and GST=0.660, D = 0.008 – 0.422 for the southern group), probably meaning more recent dispersal of L. kessleri than the two groups of L. reissneri. The parasitic and anadromous L. japonicum appeared to be closely related to L. kessleri (D=0.042 – 0.090) and the northern group of L. reissneri (D=0.163 – 0.355), which have nonparasitic and fluvial life styles, whereas the southern group of L. reissneri was greatly divergent from the other three Lethenteron species (D=0.559 – 0.926), suggesting that the former three species might be monophyletic.

Genetic differentiation ofLethenteron reissneri populations, with reference to the existence of discrete taxonomic entities

Genetic divergence in the Far Eastern brook lamprey,Lethenteron reissneri, collected from Hokkaido and Honshu Islands, Japan, was investigated by electrophoretic analysis. Two local groups, northern and southern, were distinguishable by complete allele substitutions at 11 loci, resulting in high genetic differentiation (D=0.558–0.926). In addition, no genetic evidence of hybridization between the two groups was detected in three sympatric river populations, strongly suggesting their reproductive isolation, and separate specific status of the groups. In each local group, the genetic variability within and genetic differentiation between each population was considerable (H=0.034–0.147,Gst=0.508 for the northern group; H=0.024–0.105,Gst=0.629 for the southern group), suggesting large sizes for most populations, along with considerable isolation of each population from the others, resulting in lowered gene flow between them. Based on allele distribution and the cluster pattern on a UPGMA dendrogram, the northern and southern groups could be divided into two and three subgroups, respectively, which may have resulted mainly from isolation due to geographic barriers.

Morphometric and meristic characteristics of two groups ofLethenteron reissneri

Morphometric and meristic characteristics were compared between two genetically divergent groups ofLethenteron reissneri, collected from Hokkaido and Honshu islands, Japan. Whereas significant differences were found in some morphometric and meristic characters, the ranges overlapped somewhat in all of the characters examined between the two groups. Thus, it was difficult to distinguish exactly between the two groups using only the morphological characters. In sympatric populations of the two groups, no significant differences were found in almost all of the morphometric and meristic characters examined. The two groups ofL. reissneri, which are reproductively isolated from each other, should be regarded as sibling species. Assortative mating, based on morphological features, appears unlikely as the reproductive isolating mechanism between the two groups.

Breeding season and nesting assemblages in two forms of Lethenteron reissneri, with reference to reproductive isolating mechanisms

The level of reproductive isolation and reproductive characteristics were examined in sympatric populations of the northern and southern forms ofLethenteron reissneri, in the Ushiwatari Stream, a tributary of the Gakko River, Yamagata Prefecture, Japan, to elucidate the reproductive isolating mechanisms existing between them. Extensive allozyme data showed no gene exchange between the two forms in the study area, evidence of their strict reproductive isolation from each other. The spawning seasons overlapped between the northern and southern forms, the spawning period of the latter being slightly later than that of the former. The size at maturity also overlapped between the two forms, although the southern form indicated significantly larger in average size than the northern one. These results suggested that neither seasonal reproductive isolation nor size assortative mating occurred. On the other hand, the nesting assemblages were observed to comprise only males and females of each respective form, suggesting that a recognition system restricted to each form may operate as a premating isolating mechanism.

Mature dwarf males and females of the arctic lamprey, Lethenteron japonicum

L ife-history polymorphism, such as anadromous and fluvial forms within the same species, is well known in many salmonid fishes tSvfirdson. 1971 : Johnson. 1980 Kawanabe. 1989: Yamamoto et al,, 1992). However. only a few examples have been reported in lamprey species (Berg, 1931, 1948; lwata and Hamada, 1986). The arcnc lamprey, Lethenteron japonicum Martens). is wldely distributed, occurring in Hokkaido and the northern part o f Honshu Island. Japan. the Siberian region. Sakhalin Island. the Korean Peninsula, Alaska and northern Canada (Berg. 1931. 1948: Hubbs and Potter. 1971: Choi et al., t984: lwata et al.. 1985). The species generally has a parasitic anadromous life-style and reaches over 350mm in total length (TL) at maturity (Berg, 1931. 1948: Sato. 1951: Hubbs and Potter. 197l: Iwata. 1989 ~, Berg I 193 l. 19481. however, gave a brief description on a spent dwarf male of L. japonicum (224mm TL~, from the mouth of the Kukhtui River. near Okhotsk, naming it aberatio nlgra. Iwata and Hamada (1986t also reported a single dwarf male individual of L. japonicum r t97,2 mm TL) with a maturing tesus, collected from the Assabu River. southern Hokkaido Island, They reasoned that the specimen had resided solely in freshwater and participated in reproduction with anadromous adult females. During the present study, a number of dwarf male and female L. japomcum were collected in Hokkaido and northern Honshu Island. Japan. the occurrence o f dwarf females being the first record for any lamprey species. In this paper, some morphological characteristics of these dwarf individuals are described and compared with both the dwarf male reported bv Iwata and Hamada ~1986} and sympatric anadromous adults of L. jctponicum collected at the same time. The reproduction o f dwarf individuals is briefly discussed in relation to size assortative mating. Materials and M e t h o d s

Histological Comparisons of Intestines in Parasitic and Nonparasitic Lampreys, with Reference to the Speciation Hypothesis

Abstract Histological comparisons of intestinal internal structures were made for the monophyletic lamprey group comprising parasitic Lethenteron japonicum, and nonparasitic L. kessleri and the northern form of L. reissneri, in order to verify the speciation hypothesis that the nonparasitic species have been derived from a congeneric parasitic species. In the larval stage of each species, the mucosal epithelial cells were regularly arranged around an inner layer of intestine, including the typhlosole. At the metamorphosed stage, L. japonicum possessed functional mucosal folds, reflecting an adaptive change for parasitic feeding after metamorphosis. The two nonparasitic species, in which feedings are absent after metamorphosis, also exhibited mucosal folds albeit in a degenerative condition, indicating the likely presence of functional or at least rudimentary mucosal folds in an ancestral parasitic species. This finding supports a previously advocated direction of speciation in lamprey satellite species, namely nonparasitic L. kessleri and the northern form of L. reissneri speciated from ancestral stocks of parasitic L. japonicum.

Embryonic development of the Pacific lamprey, Entosphenus tridentatus.

Abstract Embryonic development of the Pacific lamprey, Entosphenus tridentatus, from Japan is described. Egg sizes averaged 1.249 mm (longest axis) and 1.145 mm (shortest axis), the time required for hatching being 11 days at 18°C, shorter than previously reported for a lower water temperature (19 days at 15°C). Early development in E. tridentatus proceeded at a similar rate to that in other lampreys, in spite of different rearing water temperatures for the latter, indicating possible specific differences in basic developmental rates.

Patterns of Asexual Reproduction in the Fissiparous Seastar Coscinasterias acutispina (Asteroidea: Echinodermata) in Japan

Abstract The morphological and anatomical characteristics of the fissiparous seastar Coscinasterias acutispina were investigated at 23 sites around Japan in order to clarify the environmental factors determining asexual reproduction. The frequency of recent fission among the sites differed, although no significant differences were evident among the three major sea areas around Japan (the Sea of Japan, Seto Inland Sea, and Pacific Ocean), indicating that the spatial variability in incidence of fission might be determined on a local scale. Morphological observations revealed that the seastars endoparasitized by Dendrogaster okadai had not undergone fission recently, suggesting that fission was suppressed by parasitism. Stepwise multiple regression analysis of the frequency of seastars that had recently undergone fission revealed that nutrient conditions affected the incidence of fission. The relationships between the pyloric caeca index and arm length ratio at the individual level suggested that good nutritional conditions were required before the occurrence of fission. Additionally, regression analysis showed that high temperature was one of the triggers of fission in C. acutispina. Thus, the process of asexual reproduction in C. acutispina appears to be governed by intricate interactions among several factors, including parasitism by D. okadai, nutrient conditions, and high temperature.

Multiple Origins and Admixture of Recently Expanding Japanese Wild Boar (Sus scrofa leucomystax) Populations in Toyama Prefecture of Japan

Japanese wild boar (Sus scrofa leucomystax) populations have expanded drastically throughout the Japanese Archipelago in recent decades. To elucidate the dispersal patterns of Japanese wild boar in Toyama Prefecture in central Japan, we used a multi-locus microsatellite DNA analysis to determine its population structure and the degree of admixture. The deviation from Hardy-Weinberg equilibrium was detected in either total or separate regional wild boar samples from Toyama Prefecture. This result could be explained by the Wahlund effect resulting from the mixture of samples from different sources. Bayesian structure analysis, assignment test, and factorial correspondence analysis suggested that wild boars around Toyama Prefecture derive from at least two ancestral sources. The migration and possible mating of each individual may have occurred recently and continued in each geographically neighboring region. The present genetic results may be useful for prediction of future dispersal patterns of Japanese wild boar, as well as other animals in expansion.