Multiple waves of freshwater colonization of the three-spined stickleback in the Japanese Archipelago.

BACKGROUND: The three-spined stickleback (Gasterosteus aculeatus) is a remarkable system to study the genetic mechanisms underlying parallel evolution during the transition from marine to freshwater habitats. Although the majority of previous studies on the parallel evolution of sticklebacks have mainly focused on postglacial freshwater populations in the Pacific Northwest of North America and northern Europe, we recently use Japanese stickleback populations for investigating shared and unique features of adaptation and speciation between geographically distant populations. However, we currently lack a comprehensive phylogeny of the Japanese three-spined sticklebacks, despite the fact that a good phylogeny is essential for any evolutionary and ecological studies. Here, we conducted a phylogenomic analysis of the three-spined stickleback in the Japanese Archipelago. RESULTS: We found that freshwater colonization occurred in multiple waves, each of which may reflect different interglacial isolations. Some of the oldest freshwater populations from the central regions of the mainland of Japan (hariyo populations) were estimated to colonize freshwater approximately 170,000 years ago. The next wave of colonization likely occurred approximately 100,000 years ago. The inferred origins of several human-introduced populations showed that introduction occurred mainly from nearby habitats. We also found a new habitat of the three-spined stickleback sympatric with the Japan Sea stickleback (Gasterosteus nipponicus). CONCLUSIONS: These Japanese stickleback systems differ from those in the Pacific Northwest of North America and northern Europe in terms of divergence time and history. Stickleback populations in the Japanese Archipelago offer valuable opportunities to study diverse evolutionary processes in historical and contemporary timescales.

Plate reduction in southern Japanese freshwater populations of threespine stickleback (Gasterosteus aculeatus).

Adaptation to similar environments can lead to the evolution of similar phenotypes in phylogenetically independent lineages. However, the extent of parallel evolution often varies. Because such variations can be due to environmental heterogeneity among seemingly similar habitats, identification of the environmental factors that cause non-parallel patterns can provide valuable insight into the ecological factors associated with phenotypic diversification. Armor plate reduction in replicate freshwater populations of the threespine stickleback (Gasterosteus aculeatus) represents a well-known example of parallel evolution. Many freshwater populations in multiple regions of the Northern Hemisphere have reduced plate numbers, but not all freshwater populations exhibit plate reduction. In this study, we characterized plate number variation in Japanese freshwater populations and investigated the association between plate number and several abiotic environmental factors. We found that most freshwater populations have not reduced plate numbers in Japan. Plate reduction tends to occur in habitats with warmer winter temperatures at lower latitudes in Japan. In contrast, low dissolved calcium levels or water turbidity had no significant effects on plate reduction, although these were reported to be associated with plate reduction in Europe. Although our data are consistent with the hypothesis that winter temperatures are associated with plate reduction, further studies on the relationship between temperatures and fitness using sticklebacks with varying plate numbers are necessary to confirm this hypothesis and understand the factors causing variations in the extent of parallel evolution.