高橋 鉄美

Abstract The evolution of sexual dimorphism is widely acknowledged as a manifestation of sex-specific genetic architecture. Although empirical studies suggested that sexual dimorphism evolves as a joint consequence of constraints arising from genetic architecture and sexually divergent selection, it remains unclear whether and how these established microevolutionary processes scale up to the macroevolutionary patterns of sexual dimorphism among taxa. Here, we studied how sexual selection and parental care drive sexual dimorphism in cichlid fishes from Lake Tanganyika. We found that male–male competition, female choice, and maternal mouthbrooding are associated with sexual dimorphism in body length, body colour, and head length, respectively, despite strong allometric relationships between body length and head length. Within-species (static) allometry of head length on body length evolved as sex-specific responses to mouthbrooding, where females evolved higher intercepts while males evolved steeper slopes. Thus, selection to increase mouth size in mouthbrooders may have broken down and reorganized the pattern of allometric constraints that are inherently strong and concordant between sexes. Furthermore, sex-specific responses to mouthbrooding left a remarkably clear signature on the macroevolutionary pattern, resulting in a decoupling of co-evolution in parameters of static allometries between sexes observed exclusively within maternal mouthbrooders. Our study provides multiple lines of evidence that are consistent with the idea that macroevolutionary patterns of sexual dimorphism in Lake Tanganyika cichlids result from sexually divergent selection. Our approach illustrates that an examination of within-population phenotypic variance in the phylogenetic comparative framework may facilitate nuanced understandings of how macroevolutionary patterns are generated by underlying microevolutionary processes.

We conducted an investigation into ghost fishing, a phenomenon in which abandoned fishing equipment remains operational for extended durations, in the southern region of Lake Tanganyika, Republic of Zambia. The perils associated with ghost fishing have achieved international recognition; however, there exists a conspicuous dearth of scholarly investigations in particular regions, notably inland aquatic environments and developing countries. Employing scuba diving for field observations, we discovered that ghost fishing was solely evident in monofilament nylon nets, despite the presence of four different net types in the littoral area of the lake. Notably, endemic crabs Platythelphusa armata and P. maculata from Lake Tanganyika constituted 65.2% of the organisms captured by abandoned nets, with the remainder comprising various fish species. The number of captured organisms exhibited a dependence on the interplay between the length and mesh size of the abandoned nets, indicating that the risk of ghost fishing is heightened when both net and mesh size are substantial. Furthermore, we encountered predatory Spiny Eels Mastacembelus moorii ensnared in nets around their jaws and teeth. Intriguingly, the digestive tract of one of the Spiny Eels contained a partially digested cichlid species entangled in the same nets, strongly suggesting the occurrence of secondary ghost fishing.

Rhodeus ocellatus kurumeus is a small cyprinid fish endemic to the Japanese archipelago. This fish frequently hybridizes with an exotic subspecies Rhodeus ocellatus ocellatus, and non-introgressed populations of R. o. kurumeus have greatly decreased. Previous studies based on mitochondrial and nuclear DNA identified introgressed populations and inferred phylogenetic relationships, but these approaches may lead to underestimates of introgression or ambiguous results owing to the small number of genetic markers used. The present study applied double digest restriction site-associated DNA (ddRAD) sequencing to assignment tests and phylogenetic inferences. Data matrices can vary in size and content depending on the strategies used to process ddRAD sequences; therefore, we prepared 25 data matrices with various processing strategies. The assignment tests based on the 25 data matrices resulted in similar assignment patterns and provided evidence for introgression in some populations that had been identified as non-introgressed populations in previous studies. The maximum-likelihood (ML) phylogenetic trees differed in robustness and topology among the 25 matrices; interestingly, ML trees with low statistical support reflected geographical distributions of fish better than ML trees with high statistical support. These results indicate that ddRAD sequencing can detect introgression with greater sensitivity than conventional DNA markers, that ddRAD sequencing is useful for phylogenetic inference among closely related populations within a subspecies, and that statistically robust ML trees do not necessarily reflect the true phylogeny. Application of ddRAD data to conservation genetics and evolutionary history of this subspecies are also discussed.

Based on morphological and molecular analyses of a Petrochromis fish (Cichlidae) from the southern end of Lake Tanganyika, this fish is considered a taxonomic species distinct from the six known congeners. A new scientific name is proposed for this fish. A key to the seven Petrochromis species is included.

The infraorbitals (IOs) of four species endemic to Lake Tanganyika were examined and on the basis of this information and previous morphological and molecular studies, the tribe Greenwoodochromini is synonymized with the tribe Limnochromini and a new combination for Limnochromis abeelei and Limnochromis staneri is proposed: Greenwoodochromis abeelei and Greenwoodochromis staneri. The revised tribe Limnochromini, comprising 10 species belonging to seven genera, is characterized by IOs representing types G and I. The revised genus Greenwoodochromis, which consists of four species, is characterized by IOs representing type I.

分子で発見した夫婦の「絆」～ある熱帯魚でのはなし. 京都大学プレスリリース. 2011-11-23.A focus on pair bonds between males and females is fundamental to study the evolution of social organization. Because pair bonds are generally identified from direct observations of pairs that maintain physical proximity, pair bonds may have been overlooked in animals that do not exhibit such visible pairs. The Lake Tanganyika cichlid fish Xenotilapia rotundiventralis forms schools that consist of mouthbrooding and non-brooding adults in mid-water, and visible pairs are not recognized. A previous study suggested that mouthbrooding females transfer fractions of the young to males when the young become large. However, it remains a mystery whether the mating pairs maintain pair bonds so that the females can transfer the young to their mates. To answer this question, we conducted a parentage analysis using 10 microsatellite markers. The analysis showed that the mouthbrooding adults were most likely genetic fathers and mothers of the young in their mouths. This finding suggests that the female-to-male shift of young takes place between mating partners, and thus the mating pairs maintain pair bonds at least until the shift of young. The present study is the first to detect pair bonds in animals in which physical proximity has not been observed.

The dwarf morph of the Lake Tanganyika cichlid Telmatochromis temporalis uses empty snail shells as shelters and breeding sites in shell beds, in which many empty shells exist. Here, we assessed selection forces regulating body size in this fish. Field observations showed that large males tended to have a greater number of females in their territories, suggesting that sexual selection favours large males. Nonetheless, a transplant experiment suggested that male body size was limited by the ability to hide in empty shells from large piscivorous fish. In females, the number of ovarian eggs increased with body size, suggesting that fecundity selection favours large females. However, females are smaller than males. Females spawned eggs close to the apices inside the shells. The small space there would decrease the risk of egg predation by egg predators, and small body size of females may be a result of adaptation to spawn eggs in the small, safe spaces. This study provides support for the idea that male and female body sizes have been limited by different ecological pressures (predation on adult fish in males, predation on eggs in females), which has not been reported previously in any animal.

Although the cichlid fishes from Lake Tanganyika are treated as a textbook example of adaptive radiation, many taxonomic problems remain unresolved. <italic>Cyathopharynx furcifer</italic>, which belongs to the currently monospecific genus <italic>Cyathopharynx</italic>, contains two colour morphs at the southern end of the lake: one has a yellow anal fin, and the other has a black anal fin. Some books for hobbyists of ornamental fish treat these morphs as different species, but taxonomic studies have neither mentioned the existence nor addressed the status of these colour morphs. In the present paper, we analysed these two colour morphs using mitochondrial, microsatellite, morphometric, and meristic data sets. Both molecular and morphological data allowed clear discrimination between these morphs, suggesting the existence of two distinct sympatric species. Three taxonomic species have been described in this genus, and only <italic>C. furcifer</italic> is currently considered valid. Observations of type specimens of these three nominal species will be needed to determine the scientific names of these colour morphs.

The degree of lunar synchronization of spawning is thought to be related to a strategy that decreases predation on the brood in Tanganyikan substrate-brooding cichlids. Here, I examined the periodic change of ovary development in two morphs of Telmatochromis temporalis: the normal morph uses burrows under stones as spawning nests, whereas the dwarf morph uses holes within empty snail shells. The normal morph showed a significant lunar synchronization of ovary development, but the dwarf morph did not. In the normal morph, spawning prior to the full moon probably decreases the incidence of approaching brood predators and increases the guarding efficiency of parents. In the dwarf morph, however, lunar cyclic spawning may be dispensable, because the spawning nests within shells are probably highly effective for predator avoidance. These two morphs are closely related, and then will be a good model to clarify the widely observed phenomenon of lunar cyclic spawning.

Morphological dimorphism in the mouth-opening direction (‘lefty’ versus ‘righty’) has been documented in several fish species. It has been suggested that this deflection is heritable in a Mendelian one-locus, two-allele fashion. Several population models have demonstrated that lateral dimorphism is maintained by negative frequency-dependent selection, resulting from interactions between predator and prey species. However, other mechanisms for the maintenance of lateral dimorphism have not yet been tested. Here, we found that the scale-eating cichlid fish <italic>Perissodus microlepis</italic> exhibited disassortative mating, in which reproductive pairings between lefties and righties occurred at higher than expected frequency ( <italic>p</italic> &lt;0.001). A previous study reported that a lefty–righty pairing produces a 1 : 1 ratio of lefty : righty young, suggesting that disassortative mating contributes to the maintenance of lateral dimorphism. A combination of disassortative mating and negative frequency-dependent selection may stabilize lateral dimorphism more than would a single mechanism.

Lepidiolamprologus mimicus n. sp. is described from material collected along the Zambian coast of Lake Tanganyika. It is distinguished from congeners by its unique color pattern of bright yellow fins, a brownish-tan flank coloration and large, dark brown spots along the flanks, in addition to a series of meristic and morphometric characters. Lepidiolamprologus mimicus n. sp. exhibits an interesting feeding ecology, in which individuals blend into schools of their prey, yellow-finned cyprichromines, with the aid of similar coloration. This is the first instance of aggressive mimicry reported for lamprologines.

琵琶湖に生息するオオクチバスとブルーギルをカワウの胃内容物から種判別する方法を提唱する目的で歯骨の形態観察と計測を行った。両種は歯骨の背面に短尖頭の歯が2列以上あること,歯骨側面の管状の側線が5個の孔で開口すること,歯骨高が歯骨長の30%以上であることから他の琵琶湖および周辺水域に生息する真骨魚類と区別できた。またオオクチバスでは歯骨高が歯骨長の41.6%より低いのに対し,ブルーギルでは44.0%より高いことから識別できた。両種の歯骨長・高と体長の関係式も求めた。

琵琶湖産アユとワカサギをカワウの胃内容物から種判別できるようにする目的で尾鰭骨格の観察を行った。両種は尾部棒状骨に下尾骨が癒合せず,2-5個の尾鰭椎前椎体に神経棘が関節することから他の琵琶湖に生息する魚類と区別できた。またアユは尾鰭椎前第3および第4椎体の神経棘が筒状の構造を持ち,先端に軟骨を有すること,そして上尾骨が2本で後方の下端が2叉するか,もしくは3本であることにより,ワカサギと明瞭に識別することが出来た。また両種の尾鰭骨格と体長の関係式も求めた。