Cryptic diversity of marine gastropod Monodonta labio (Trochidae): did the early Pleistocene glacial isolation and sea surface temperature gradient jointly drive diversification of sister species and/or subspecies in the Northwestern Pacific?

Abstract

Formation of glacial refugia during the Pleistocene climatic oscillations has been put forward to elucidate the diversification of marine organisms in the north‐western Pacific. The marine gastropod Monodonta labio is one of the most common species in the Northwestern Pacific and possibly possesses cryptic diversity. Here, we investigate the phylogeographic pattern of this species to test the potential mechanisms driving its diversification in the Northwestern Pacific. Genetic information for two mitochondrial genes (Cytochrome oxidase subunit I and 16S rDNA) and one nuclear gene (internal transcribed spacer 1) was acquired to detect genetic structuring and to reconstruct the gastropod's phylogenetic history. Our results revealed that M. labio is comprised of five main clades, and divergence time estimates place their cladogenesis as corresponding to the initiation (c. 2.5 mya) and intensification (c. 0.9 mya) of large‐scale Northern Hemisphere ice sheets. The early and middle Pleistocene divergence times are consistent with the emergence times of the Dongshan land bridge, which would seperate the ancient East China Sea and the ancient South China Sea forming two potential refugia. In addition, the deep trough in the Qiongzhou Strait would possibly act as another potential refugium with the uplift of the Qiongzhou Strait at mid‐Pleistocene. This study suggests that the current genetic architecture of M. labio is probably correlated with glacial isolation and sea surface temperature gradient. We also put forward the possibility that these factors were probably an important driver for the diversification of sister species or subspecies of other taxa in the Northwestern Pacific.

Shell variations in the gastropod,Monodonta labio, in the North-western Pacific: the important role of temperature in the evolution process

“…Revealing patterns of phenotypic variation is the first step towards understanding the mechanisms of evolution and speciation. In this study, a pattern of divergence in shell shape among different populations of Monodonta labio was identified, with the most marked divergence in shell shape between two major genetic groups (Clade A + B + C and Clade D + E of Zhao et al ., 2017). In a previous study (Takenouchi, 1985), morphological diversity of this species was also reported in Japan.…”

“…Specimens of Monodonta labio shells used in this study belonged to the DNA-sequenced individuals used in the prior genetic study (Zhao et al ., 2017); specimens with damaged shells were excluded. The specimens used in this study were randomly collected from different microhabitats (rocky, cobble and boulder shores) in all populations at summer.…”

“…Linear discriminant analysis (LDA) was performed on the first two principal components of the PCA against the main genetic groups (Clade A + B + C and Clade D + E of Zhao et al ., 2017). The dataset containing all individuals was split into training (143 individuals) and test sets (144 individuals), and LDA model was trained using the training set.…”

“…To test the efficacy of all six measured characters in classifying the shells, a regression analysis was performed against the main genetic groups (Clade A + B + C and Clade D + E of Zhao et al ., 2017). As measured characters included both Boolean and continuous variables, a logistic regression model was used to explain the relationship between measured characters and genetic groups.…”

“…However, the sampling localities were limited and did not cover all the variable environmental factors of this region. Furthermore, whether the variations in shell shape of M. labio reflect genetic differences remains untested, although five genetically different lineages have been revealed in a phylogeographic study using mitochondrial sequence data (Zhao et al ., 2017). In addition, the presence of two distinct morphotypes, M. labio labio and M. labio confusa (e.g.…”