Effect of founding events on coat colour polymorphism ofApodemus speciosus(Rodentia: Muridae) on the Izu Islands

Abstract

To infer the evolutionary mechanism of phenotypic variation among isolated island populations, we investigated coat colour and genetic variation in the large Japanese field mouse (Apodemus speciosus) on the Izu Islands (Ohshima, Niijima, Kouzushima, and Miyakejima). Coat colour in the most remote population (Miyakejima) was unique and significantly darker than that in the other populations. Ohshima that is closest to the source population showed variation in coat colour within its population. Phylogeographical analyses using mitochondrial and microsatellite markers suggested that the island populations (except Kouzushima) were founded sequentially from the closest Ohshima to remote Niijima and Miyakejima during or before the penultimate interglacial period. Secondary gene flow from the source population was rare and occurred only for the closest (Ohshima) population. In addition, we found that an amino acid mutation in the Agouti signalling protein gene (Asip) was associated with coat colour variation among the island populations. The mutation was rare in the source population but completely fixed in the Miyakejima population. The phenotypic and genetic variation suggested that severe reduction of genetic variation and changes in allele frequency as a result of sequential colonization (i.e. the founder effect) had significant effects on colour polymorphism. The findings of the present study suggest that the founder effect, in addition to natural selection, facilitated the morphological changes below the species level over a relatively long time scale.

Estimation of Evolutionary Rates of Mitochondrial DNA in Two Japanese Wood Mouse Species Based on Calibrations with Quaternary Environmental Changes

“…The sequences of 15 A. argenteus individuals were newly determined in this study; the remainder were those used in previous studies (Serizawa et al, 2000;Suzuki et al, 2004Suzuki et al, , 2015. The Cytb sequences of A. speciosus used in this study (90 individuals) were obtained from databases (e.g., Suzuki et al, 2015), with special emphasis on Sado (Tomozawa et al, 2010) and Izu Islands (Tomozawa et al, 2014). Newly obtained individuals were captured with permission from respective prefectural governments to use them only for academic research.…”

“…In drawing the curves of the evolutionary rate, we assumed interactions between islands at the glacial maxima of 100,000-year intervals. However, given that population expansions during the warming periods associated with glacial maxima are the most likely drivers of multiple branching in the mtDNA lineages in the wood mice of temperate origin (Tomozawa et al, 2014;Suzuki et al, 2015), assuming that the branching patterns have been shaped by expansion events in the interglacial warm periods immediately after the glacial maxima of 100,000-year intervals, there may be an alternative explanation for the divergence in the data on the remote islands. Setting expansion-based calibration points, namely the initiation times of the interglacial warm periods (Jansen et al, 2007), provides alternative plotting points for the genetic distances between the insular lineages ( Fig.…”

“…Among murine rodents, Eurasian wood mice (genus Apodemus) would be ideal subjects. There have been a number of phylogeographic studies on Apodemus species, and these have shown that their spatial genetic architectures have been structured without anthropological influence (Suzuki et al, 2004;Tomozawa and Suzuki, 2008;Tomozawa et al, 2010Tomozawa et al, , 2014; but see Herman et al, 2016) and that their evolution has been spurred by climatic changes and habitat topography (Hsu et al, 2001;Michaux et al, 2004Michaux et al, , 2005Herman et al, 2016).…”

“…Two species of Apodemus are endemic to the Japanese Islands. The large wood mouse A. specious is the most common rodent in Japan, and as such inhabits small remote peripheral islands, such as Sado Island, Izu Islands, Tsushima Islands and Satsunan Islands, which are currently separated from one another and from the four main islands of Hokkaido, Honshu, Shikoku and Kyushu by deep sea (e.g., > 120 m) (Tada et al, 1999;Tomozawa et al, 2010Tomozawa et al, , 2014. Its dispersal events are thought to have been associated with glacial 100,000-year cycles (Tomozawa and Suzuki, 2008;Tomozawa et al, 2010Tomozawa et al, , 2014Suzuki et al, 2015) and may thus be useful as calibrators for assessing evolutionary rates.…”

Peer Review #2 of "Effect of isolation on coat colour polymorphism of Polynesian rats in Island Southeast Asia and the Pacific (v0.2)"

“…Mundy et al, 2004), In mice, Agouti further regulates the differences between dorsal and ventral pigmentation and may be responsible for the switching off of pigment production altogether (Vrieling et al, 1994). Dark buff or rufous undersides (contrary to the usually white ventral pelage) were observed in the most remote population of the large Japanese field mouse, Apodemus speciosus (Temminck, 1844), on the Izu Islands (Tomozawa et al, 2014), explained as the result of loss of genetic variation and changes in the frequency of an allele for a mutation in Asip. Laboratory mice with such mutations showed enhanced pigmentation (yellow pheomelanin) of the ventral pelage (Vrieling et al, 1994), similar to the pattern (dark buff with blackish tail) observed in the large Japanese field mouse on Miyake island (Tomozawa et al, 2014).…”