Trace of outbreeding between Biwa salmon (Oncorhynchus masou subsp.) and amago (O. m. ishikawae) detected from the upper reaches of inlet streams within Lake Biwa water system, Japan

Abstract

The establishment of fluvial fish populations from anadromous populations by natural or artificial barriers obstructing migration is a good research subject to study life history plasticity. Biwa salmon, Oncorhynchus masou subsp., a salmonid fish endemic to the Lake Biwa water system, exhibit life history variation (e.g., mature stream-resident males) in addition to a typical lacustrine life history type, indicating potential adaptations of life histories in response to emergence of barriers. Currently, fluvial populations that are morphologically similar to both stream-resident Biwa salmon and amago, the fluvial red-spotted masu salmon, Oncorhynchus masou ishikawae, are found upstream of dams which were constructed in the inflowing rivers of Lake Biwa. However, it is unknown whether they are Biwa salmon or amago. To explore that, the genetic characteristics of nine fluvial populations were investigated through AFLP and mtDNA analyses. Bayesian admixture analysis based on the AFLP data revealed that three fluvial populations were admixed populations between Biwa salmon and amago. In addition, a Biwa salmon mtDNA haplotype was detected in some individuals from three populations. However, no genetically pure fluvial populations of Biwa salmon were found, indicating no life history plasticity in this subspecies, and thus hybridization with amago boosted the ability of this subspecies to establish fluvial populations. Nevertheless, other scenarios, such as hybridization after establishment of fluvial populations of Biwa salmon, are also possible. The latter hypothesis could be supported by the fact that amago did not inhabit the river before emergence of barriers. However, a significant genetic population structure was found only in amago, suggesting that this subspecies is native to the Lake Biwa water system. But the possibility that multiple sources of amago have been released into rivers cannot be excluded. Therefore, further studies on the relationships between amago populations in the upper reaches of the Lake Biwa water system and other populations in the surrounding areas of the Lake Biwa water system are needed to clarify the origins of the admixed populations.