4. Discussion
4.1 Life cycle and distribution
V. micado has two ways to get through the winter, one group prefer to nymph diapause, the other group choose to egg diapause (He and Takeda, 2013). Adults of egg diapause population were found from July to October (Shandong, Heilongjiang, Henan, Guizhou and Yunnan), and nymphs were from July to August (Shandong and Yunnan). The individuals with nymph diapause, as we found, were adults in March (Hainan), April (Guangxi), June (Hubei and Zhejiang) and July (Guizhou). The nymph were found in the spring or the later autumn, such as March (Hainan), April (Guangxi), August (Guizhou), September and October (Zhejiang).
Our results indicated that V. micado was transferred to America, which is consistent with this result (Bowles, 2018). V. micadowas first found in the North America since 1959 (Alexander and Walker 1962), then they were widely distributed around eastern and southeastern America (Walker 1977; Peck et al . 1992). The recent research indicated the cricket had dispersed both northwards and westwards (Bowles, 2018). Our results revealed they might be immigrated from China and Japan, and belongs to the egg diapause populations. Their distribution range in the United States might continue to expanding due to their adaptation to dry and cold areas, for example, they were widely distributed in the northeastern China. Limited to sampling size, it is unknown if there are nymph populations in North America. The mitochondrial fragment COI could be easily used to check the origin ofV . micado for prevention of invasion. The similar latitude and climate make the species orginated from North America easy to settle down to China, such as Corythucha ciliata, Homalodisca coagulata and Anopheles quadrimaculatus (Li et al ., 2007; Wang et al ., 2008; Zhang et al ., 2008) and vice versa. Similarly,V. micado was easy to adapt to the environment of North America. Although it has not been found that this cricket has destructive damage to human and nature, the impact to system of the new colony is unknown. Besides, although sampling size was small, individuals in Vietnam and Cambodia might be native due to the ancient haplotype.
The wide sampling in Asia enabled us to find the distribution of two groups and compare differences between them. The egg diapause population lives in both the north and south of the Yangtze River. Based the COI, the NE group contained the populations lived in north of the Yangtze River regions as well as those in Shanghai and Zhejiang. These populations have the ancient haplotypes and have not migrated previously and recently through the values of Tajima’ D. The SE group is distributed in the south of the Yangtze River regions as well as those in Hubei, Jiangsu and Shandong. The haplotypes were the tips of gene tree in Shandong and Jiangsu, maybe the SE group was introduced. However, there was ancient haplotype in Hubei. Limited to the sample size, it is uncertain whether the three populations migrated before. The SN group included the populations lived in the south of Yangtze River and HBWH (Hubei). The two individuals are the ancient haplotype in Hubei, but it is also uncertain whether there was a migration before. Based on CytB, all individuals were divided into two groups, NC (northern China) group contains the populations lived in the north of the Yangtze River regions as well as those in Shanghai and Zhejiang, in which there is ancient haplotype. SC (southern China) group contains the populations lived in the south of the Yangtze River regions as well as those in Hubei, Anhui, Shandong, in which there are both tips of gene tree, and these individuals may immigrate into the north of the Yangtze River artificially or naturally. This phenomenon with two life histories bounded by the Yangtze River could be seen between Teleogryllus emma and T. occipitalis (He et al ., 2017).
According to the distribution of species, there is a broad faunal transition zone in the Quaternary between the Palearctic and Oriental realm in China (Zhang, 2002) and no strict biogeographic division (Norton et al ., 2011). However, a map of zoogeographic regions generated by phylogenetic relationships of species shows that there are three realms containing Palearctic realm, Sino-Japanese realm and Oriental realm (Holt et al., 2013). However, our results were basically consistent with an earlier suggestion of Wallace (Wallace, 1876). The Yangtze River is the biogeographic boundary between the Oriental and Palearctic. Although it is not strictly defined by the Yangtze River, Shandong, Jiangsu, Shanghai and Zhejiang regions are the intersection of groups. Those are the broad transition zone for V. micado . When the cool climate shift occurred in the mid-Pleistocene, the northern individuals immigrate southwards. When the glaciation in the southwestern mountain, the native residents immigrate eastwards the refuge, but some remained and scattered. The analysis is as following.
4.2 Divergence time and induced causes
Phylogenetic analysis and haplotype network have well supports thatV. micado has diversified to two main lineages (NE and SE+SN) and two modes of life cycle (egg diapause and nymph diapause), and they are evolved independently. There are three main questions. The first question concerns the divergence of V. micado . Which important events triggered them dividing to two main clades (NE and SN+SE)? The second question is which climate shifts led them had developed two modes of life cycle? The third question is about the sympatric co-existence of two modes of life cycle of V. micado , what made them differentiate?
The time estimation analysis is the most reliable criterion to confirm which important events or climate shifts result in the divergence of theV. micado . Although the rate of CytB is bigger than COI (Brower and DeSalle, 1998; Gray et al ., 2006), the individuals lived in south Asia have not diversified based on CytB. Thus, the divergence in COI occurred early than CytB after the climate shifts, and COI was selected to estimate the time of diversification.
Climate changes during the Quaternary have great effects on species, for example, climate oscillation occurred during Pleistocene has affected the distribution of species Gymnocarpos przewalskii in Northwestern China (Jia and Zhang, 2019), the genetic divergence in species as Petrogale penicillata in southeast Australia appear to date to the mid-Pleistocene (Hazlitt et al ., 2014) and even contributed to speciation of arctic-alpine Campanula occurred in mid-Pleistocene in western North America (DeChaine et al ., 2014). According to the researches about Quaternary glaciations in Asia, the climatic transformation of Asia occurred in 0.8 Ma BP was closely related to Mid-Pleistocene Transition (MPT), which was called due to glacial-interglacial cycles from a 41 ka to 100ka dominant frequency (Pisias et al ., 1981; Ruddiman et al ., 1989; Raymoet al ., 1998), and the Kunhuang (Kunlun Huanghe) movement of 0.9 Ma BP, (Li et al ., 2001; Li et al ., 2004). Since 800 ka BP, there were 8 complete glacial-interglacial cycles with a 100-ka dominant frequency (Shackleton and Opdyke, 1977). During these global glacials after the Kunhuang movement, glaciers developed gradually in the various part of the Qinghai-Tibet plateau (Wu et al ., 1999; Yao et al ., 2000; Zheng, 2000; Zhou et al ., 2002). Mountains lower than 2000 m in the East China were no glaciers during the middle Pleistocene (Shi et al ., 1987). The deposit sediment and the current distribution of the extant land species were related to the geographical and climatic shifts since the Quaternary. The most recent Vermiculated Red Soil (VSR) in southern China, an indication of extremely warm and humid conditions, was formed about 0.85 Ma BP due to the strengthen summer monsoon in the middle Pleistocene (Yang et al ., 1996; Qiao et al ., 2003) and the summer monsoon also strengthened in northern China (Guo et al ., 1998). However, subtropical zone reached at 42°N before the middle Pleistocene, and retreated southwards after the mid-Pleistocene (Liu and Ding, 1983). Many species immigrated southwards, including boreal, thermophilic and humid-preferring fauna. The species adapted to the new environment, and some remained and scattered in the locality during the Quaternary glaciation (Zhang, 2004). The current distribution of these species revealed the overall trend after the climate oscillations. The divergence time between northern and southern individuals was dated to 0.79 Ma, which was basically consistent with this time, thus, it is possible that the life history of the individuals lived in northern China changed under the tendency of change towards cooling climate. During the MPT, mid-Pleistocene Homo (MPH) (Bae, 2010; Wu and Poirier, 1995) in East China adapted to the diverse and various climate, distributing from the temperate to subtropical zone (Guo et al ., 2018; Kong et al ., 2019). The climate shifts in MPT induced the diversification and distribution of species, and we observed that this cricket is common around the habitats of human, but not in the wild. It is possible that human behavior influenced the distribution of the cricket to some extent after the mid-Pleistocene.
The earliest Yunshanping moraine in Yulong mountain (Yunnan province) was dated to 592.6 ± 118.5 ka BP (Zheng, 2000). Yao conducted dating studies on the red moraine in the same mountain, the ages of several samples range from 500 to 600 ka BP (Yao et al ., 2000). The second differentiation occurred during ~ 0.50 Ma between southern individuals with egg and nymph diapause populations, and our results showed the egg diapause population might originated from Yunnan. Thus, the individuals with nymph diapause changed to get through the winter with egg due to the dry and cool climate.
The behavioral and genetic changes in the southern residents correspond to the environmental transitions. The genetic diversity indices and the phylogenetic analysis are the reliable evidence to support that the differentiation among sympatric co-existence of V. micado with two modes of life cycle has occurred for a long time. The divergence between them is maintained in the absence of obvious environmental difference and barriers to gene flow, such as the same photoperiod, temperature, humidity and habitats, while the genomic underpinning of ecological speciation often appear to have been found to be the result of a long period of allopatry (Bernatchez and Dodson, 1990; Federet al. , 2003; Gray et al ., 2006; Kuehne et al . 2007; Foote and Morin 2015; Lucek et al ., 2018). The egg diapause population might origin from Yunnan colonized the new habitat. The individuals lived in the same locality may choose differently suitable habitats to their own life cycle.
It should be noted that there are still parts to be improved in our research. First, the range of the sampling time in the tropical region of China should be increased to further figure out if there was the egg diapause population. Second, are genes (COI and CytB) and behavior differentiation (different modes of life cycle) just two independent responses to the dramatic environment? Has the efficiency of the function of COI changed in individuals with two modes of life cycle? Or maybe it’s just due to the founder effect. As this research shows, a large of proportion of genetic variation is originated from the founder effect rather than natural selection in human populations (Ramachandranet al ., 2005).