Kazunori Yamahira

Induction of Female-to-Male Sex Reversal by High Temperature Treatment in Medaka, Oryzias latipes

Abstract Medaka, Oryzias latipes, has a firm XX-XY sex-determining system with the sex-determining gene, DMY, on the Y chromosome. However, previous studies have suggested that high water temperature might affect sex determination in Medaka. In the present study, the influence of high water temperature on sex reversal was examined. Fertilized eggs of two inbred strains of Medaka were developed at high water temperature (32°C) until hatching. The hatched fry were kept at normal water temperatures (27°C) until adulthood, and the phenotypic and genotypic sex was examined. As a result, 24% (N=105) and 50% (N=36) of XX fish developed a male phenotype in the Hd-rR and HNI inbred strains, respectively. These XX sex-reversed males had a normal testis and were fully fertile. On the other hand, all XY fish were male in the both strains. These results demonstrate that high water temperatures can induce XX sex reversal and that elevated water temperatures during the embryonic stage is a simple and useful method for getting XX males in Medaka.

INTER- AND INTRAPOPULATION VARIATION IN THERMAL REACTION NORMS FOR GROWTH RATE: EVOLUTION OF LATITUDINAL COMPENSATION IN ECTOTHERMS WITH A GENETIC CONSTRAINT

Abstract In ectotherms, lower temperatures in high-latitude environments would theoretically reduce the annual growth rates of individuals. If slower growth and resultant smaller body size reduce fitness, individuals in higher latitudes may evolve compensatory responses. Two alternative models of such latitudinal compensation are possible: Model I: thermal reaction norms for growth rates of high-latitude individuals may be horizontally shifted to a lower range of temperatures, or Model II: reaction norms may be vertically shifted so that high-latitude individuals can grow faster across all temperatures. Model I is expected when annual growth rates in the wild are only a function of environmental temperatures, whereas Model II is expected when individuals in higher latitudes can only grow during a shorter period of a year. A variety of mixed strategies of these two models are also possible, and the magnitude of horizontal versus vertical variation in reaction norms among latitudinal populations will be indicative of the importance of “temperature” versus “seasonality” in the evolution of latitudinal compensation. However, the form of latitudinal compensation may be affected by possible genetic constraints due to the genetic architecture of reaction norms. In this study, we examine the inter- and intrapopulation variations in thermal reaction norms for growth rate of the medaka fish Oryzias latipes. Common-environment experiments revealed that average reaction norms differed primarily in elevation among latitudinal populations in a manner consistent with Model II (adaptation to “seasonality”), suggesting that natural selection in high latitudes prefers individuals that grow faster even within a shorter growing season to individuals that have longer growing seasons by growing at lower temperatures. However, intrapopulation variation in reaction norms was also vertical: some full-sibling families grew faster than others across all temperatures examined. This tendency in intrapopulation genetic variation for thermal reaction norms may have restricted the evolution of latitudinal compensation, irrespective of the underlying selection pressure.

The role of intertidal egg deposition on survival of the puffer, Takifugu niphobles (Jordan et Snyder), embryos

The puffer, Takifugu niphobles (Jordan et Snyder), is an intertidal spawner; the spawning linkage with the tidal cycle makes it possible for the puffer to spawn at the upper intertidal zone. To test whether the intertidal spawning is an adaptation to maximize egg survival, the location of egg deposition and the most suitable location for egg survival were determined. An egg-trap experiment indicated that eggs were scattered around the spawning zone and never moved downwards into the lower intertidal zone. The eggs were stranded under pebbles of the upper intertidal zone, and developed there. Some of the stranded eggs died apparently because of high temperatures and desiccation during periods of exposure. An egg-transplantation experiment indicated that throughout the spawning season, mortality of eggs by physical stress was lower in the lower intertidal zone (where eggs were never found naturally) than in the upper zone. Also, the loss rate due to predation or wave action did not differ among the tidal zones. These facts suggest that the upper intertidal spawning linked with the tidal cycle does not maximize egg survival, and that the puffer should spawn during lower tides in the tidal cycles.

Intra- and Interspecific Latitudinal Variation in Vertebral Number of Menidia spp. (Teleostei: Atherinopsidae)

Abstract Vertebral number is used frequently as a diagnostic species character in fishes, but it varies not only among but also within species. Much of the variation in vertebral number is associated with geographic gradients such as latitude, and it likely reflects both environmental and genetic influences on phenotype. Hence, comparison of intra- and interspecific geographic variation and its genetic basis in closely related species may contribute not only to taxonomy of the group concerned but also to understanding how such differences evolve. In this study, we examine intra- and interspecific latitudinal variation in vertebral number of two geographically contiguous, closely-related species: the Atlantic Silverside Menidia menidia (a northern species) and the Tidewater Silverside M. peninsulae (a southern species). In accordance with Jordans rule, vertebral numbers of wild individuals increased with latitude within and among species. Mean vertebral number varied along a common linear trajectory both within and among species and could be predicted with very high accuracy from latitude alone (r = 0.98), indicating that vertebral number is not a diagnostic species character in Menidia. Common-garden experiments revealed that this latitudinal cline persists across all temperatures, demonstrating a strong genetic basis within and among species. The continuous variation throughout the entire geographic range across the species suggests that natural selection and adaptation is the most likely explanation, and that populations and species are free to evolve any vertebral number required in each environment.

Variation in juvenile growth rates among and within latitudinal populations of the medaka

In ectotherms, lower temperatures at high latitudes would theoretically reduce annual growth rates of individuals. If slower growth and resulting smaller body size reduce fitness, individuals at high latitudes may evolve compensatory growth. This study compares individual growth rates among and within 12 latitudinal populations of the medaka (Oryzias latipes). Growth rates during juvenile stage were measured in a common, temperature-controlled (28°C) environment. The results revealed that juvenile growth rates differed significantly among the populations. Growth rates were, moreover, significantly correlated with latitudes of source populations, such that higher-latitude individuals grew faster. Significant variation in growth rates among full-sib families within populations was also demonstrated. The results strongly suggest that higher-latitude O. latipes have acquired a greater capacity for growth as an adaptation to shorter growing seasons (which would reduce annual growth rates), thus refuting probability processes, i.e., genetic drift, founder, or bottleneck effects, as a cause of the among-population variation.

Interpopulation Variability in Temperature-Dependent Sex Determination of the Tidewater Silverside Menidia peninsulae (Pisces: Atherinidae)

Abstract The existence of temperature-dependent sex determination (TSD) and its latitudinal variability among populations were examined experimentally in Menidia peninsulae. Along the Florida coast, TSD was detected only in a northern population (Apalachee Bay: 29°55′N), wherein the proportion of females declined dramatically with increasing rearing temperature. In lower-latitude populations (New Smyrna Beach: 29°05′N, Vero Beach: 27°46′N, and Grassy Key: 24°45′N), however, offspring sex ratios were temperature-independent and approximated 1:1 at all temperatures examined (17–32 C), indicating genetic sex determination (GSD). The shift from TSD to GSD with decreasing latitude in M. peninsulae occurs coincidentally with a shift from annual to semiannual breeding and life cycle patterns. Combined with the previous studies of the northern congener Menidia menidia (∼32–47°N), the level of TSD in the genus Menidia is generally highest at intermediate latitudes (∼32–38°N), whereas nearly pure GSD is displayed at the northern and southern extremes. This pattern of latitudinal variation across two species confirms that TSD is adaptive in Menidia only in annual life histories wherein temperature during larval development serves as an accurate predictor of length of growing season and hence relative adult body size.

Combined effects of tidal and diurnal cycles on spawning of the puffer, Takifugu niphobles (Tetraodontidae)

SynopsisSpawning in the pufferTakifugu niphobles was observed at Tomioka in western Kyushu, Japan, and the factors affecting the spawning time were estimated using a multiple regression analysis. Spawning in this puffer occurred in the intertidal beach during evening rising tides around the full and new moons. Both the time of commencement and ending of a days spawning were related mainly to the tidal cycle, and occurred later when the high tides occurred later. However, the termination of a days spawning was also affected by the diurnal cycle; spawning ended earlier when sunset was earlier. Besides, a days spawning tended to be concentrated within a shorter time span when the interval between the commencement of spawning and the sunset was shorter. Thus the spawning time of the puffer is regulated not only by the tidal but also by the diel cycle, and it is suggested that the adaptive significance of the spawning reactions to both cycles should be examined separately.

Age specific food utilization and spatial distribution of the puffer,Takifugu niphobles, over an intertidal sand flat

SynopsisSpatial distribution and feeding habits of the puffer fish,Takifugu niphobles, were studied to understand its mode of resource utilization in an intertidal sand flat at Tomioka in the western coast of Kyushu Island, Japan. The transect survey during high tides showed the presence of young puffers near the shoreline, while adults were mainly found in the offshore area of the sand flat. The gut contents analysis showed that although the young puffers foraged more frequently on organisms dwelling in the inshore area than the adults, both puffers fed mainly on the ghost shrimpCallianassa japonica inhabiting the entire sand flat. There was no apparent difference in the size of consumedC. japonica between the young and adult puffers. However, comparison of size structures of ghost shrimps in the field and in the gut contents showed that the young puffers foraged both from inshore and offshore areas of the sand flat, while the adults foraged mainly in the offshore area. This suggests that young puffers move more frequently over the sand flat with tides than adult ones. This pattern of trophic separation among size classes may reflect environmental features of the intertidal sand flat with large tidal fluctuations.

An acquisition trade-off with fast growth in a fish, the medaka Oryzias latipes: why do low-latitude ectotherms grow more slowly?

In various ectotherms, it is well-known that high-latitude individuals grow faster at any temperature than do low-latitude individuals as an adaptation to shorter growing seasons. The submaximal growth of low-latitude ectotherms implies that there exist trade-offs with fast growth. Although there is accumulating evidence for latitudinal variation in growth capacity, however, few studies have demonstrated these trade-offs. In this study, using a freshwater fish Oryzias latipes as a model organism, we tested whether there is an acquisition trade-off in which the higher growth capacity of high-latitude individuals is realized by foraging more intensively, despite the risk of predation. Foraging experiments revealed that higher-latitude O. latipes juveniles captured more prey per unit time than lower-latitude juveniles at any temperature. In addition, predation experiments revealed that higher-latitude juveniles were more frequently killed by dragonfly larvae. Observations of swimming behaviors demonstrated that the higher vulnerability to predation in high-latitude O. latipes is probably caused by lower steady- and burst-swimming performances due to the larger meal size, and partially by reduced caution due to a greater willingness to forage. We conclude that acquisition trade-offs may be the primary constraint on growth evolution across latitudes among fishes. This implies that growth capacity evolves in response not only to the length of the growing season but also to predation pressures.

Origin and intra-island diversification of Sulawesi endemic Adrianichthyidae

Although the family Adrianichthyidae is broadly distributed throughout East and Southeast Asia, 19 endemic species, over half of the family, are distributed in Sulawesi, which is an island in Wallacea. However, it remains unclear how this Adrianichthyidae biodiversity hotspot was shaped. In this study, we reconstructed molecular phylogenies for the Sulawesi adrianichthyids and estimated the divergence times of major lineages to infer the detailed history of their origin and subsequent intra-island diversification. The mitochondrial and nuclear phylogenies revealed that Sulawesi adrianichthyids are monophyletic, which indicates that they diverged from a single common ancestor. Species in the earliest branching lineages are currently distributed in the central and southeastern parts of the island, indicating that the common ancestor colonized Sula Spur, which is a large promontory that projects from the Australian continental margin, from Asia by oversea dispersal c.a. 20Mya. The first diversification event on Sulawesi, the split of the genus Adrianichthys, occurred c.a. 16Mya, and resulted in the nesting of Adrianichthys within Oryzias. Strong geographic structure was evident in the phylogeny; many species in the lineages branching off early are riverine and widely distributed in the southeastern and southwestern arms of Sulawesi, which suggests that oversea dispersal between tectonic subdivisions of this island during the late Miocene (7-5Mya) contributed to the distributions and diversification of the early branching lineages. In contrast, most species in the lineages branched off later are endemic to a single tectonic lake or lake system in the central Sulawesi, suggesting that habitat fragmentation due to the Pliocene collisions (c.a. 4Mya) among the tectonic subdivisions was the primary factor for diversification of the late branching, lacustrine lineages. Adrianichthys and some Oryzias in a certain late branching lineage are sympatric in Lake Poso, which indicates multiple colonizations of these distinct lineages into this tectonic lake. Thus, the diversification of Sulawesi adrianichthyids largely reflects the complex geological history of this island.