要約のみTohoku University千葉聡課

Kagawa Osamu

Tohoku University博士（生命科学）博士学位論文 (Thesis(doctor))要約のみthesi

Kagawa, Osamu

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Diversification through specialization in thealgae covering molluscan shells著者 Kagawa Osamu学位授与機関 Tohoku University学位授与番号 11301甲第20490号URL http://hdl.handle.net/10097/00135007     博⼠論⽂（要約）   Diversification through specialization in the algae covering molluscan shells （⾙殻に付着する藻類の特殊化を介した多様化）       令和３年度 東北⼤学⼤学院⽣命科学研究科 ⽣態発⽣適応科学専攻 ⾹川 理     Ecological specialization is an important concept in understanding the creation and maintenance of biodiversity. Biological interactions promote ecological specialization and then lead to reproductive isolation between specialists with different ecological characteristics. In particular, symbiosis, in which species with different evolutionary histories live with each other as the partner, often has close interactions and promotes speciation through symbiont-host specialization. In addition, several symbionts can further diversify by undergoing host shift, in which they switch host use to new hosts. However, how the symbiont specializes to the host and subsequently diversify is not well understood in most symbiotic systems.  Here I focused on the green alga Pseudocladophora conchopheria, which grows on the shells of an intertidal gastropod Lunella correensis. Previous studies showed that this alga has host specificity, growing only on the shell of L. correensis, suggesting that a close symbiotic relationship. However, their symbiotic interactions and evolutionary history are not well understood. I first tested the hypothesis that the alga provides the host snail with the benefit of reducing the heat stress of the host during low tide (Chapter 2). As the result, heat exposure experiments indicated that the attachment of wet P. conchopheria reduced heat stress of the host snail through heat of vaporization. Therefore, P. conchopheria would provide benefits for L. correensis. Also, P. conchopheria showed a conditional symbiosis in which the proportion of attachment depended on the environment. Thus, focusing on the fact that the relationship between the alga and the snail is well recognized by anglers or naturalists, I conducted a nationwide survey by citizen science via social media to investigate environmental factors that contribute to the establishment of symbiosis between the alga and the snail (Chapter 3). The results showed that the proportions of the algae on the shells increased with the size of the settlement substrate of the snails. Lunella correensis dives into the mud or sand at low tide in mud or sandy shores, suggesting that this behavior reduce heat stress. In contrast, L. correensis inhabiting in rocky shores may be reducing heat stress by attaching P. conchopheria on the shell. On the other hand, interestingly, there was no correlation between the proportion of the algae and temperature. A previous study suggested that P. conchopheria is neutral to the host snail. Considering that some algae are harmful to  their hosts and are generalists, this result implies that the algae can adhere to shells in cooler areas where the function of heat stress reduction is less effective. Therefore, these positive and neutral interactions may have contributed to the specialization of shell utilization by P. conchopheria. In the course of my survey, I found that, contrary to previous findings, P. conchopheria sympatricaly uses multiple intertidal gastropods other than L. correensis as hosts (Fig. 1). Did these algae specialize to their respective hosts and divergent genetically? To clarify this issue, MIG-seq, a genome-wide genetic analysis, was conducted (Chapter 4). The phylogenetic analyses showed that the algae were completely, or incompletely, genetically differentiated between host uses sympatrically. For example, in Kunetsu, located Amami Islands, where the algae use Lunella granulata and Japeuthria cingulata sympatrically as hosts, there was complete genetic differentiation and no current gene flow between the host use. The ancestral trait reconstructions revealed that the ancestor of the different host-use populations used the shell of L. granulata. Furthermore, the demographic estimation revealed that speciation with gene flow occurred between these host uses. Thus, these results implied that speciation via host shift occurred, and that host-shift speciation would proceed gradually with gene flow.  In conclusion, positive and neutral interactions in P. conchopheria may have triggered specialization to shells and shifts to various hosts. Pseudocladophora conchopheria underwent host shifts, gradually establishing host specificity and reproductive isolation, leading to phylogenetic diversification. The symbiotic interactions with the host and mechanisms of diversification in P. cochopheria were clarified using by using ecological, phylogenetic and genetic methods. I emphasize that this study provides insight into understanding in the role of interactions and the speciation continuum in the ocean symbiosis.     Figure 1: A indicates a photo of the green alga, Pseudocladophora conchopheria growing on the shell of Lunella correensis. B indicates P. conchopheria growing on the shells of various host gastropods. From left to right, are Lunella correensis, Batillaria attramentaria, Batillaria multiformis, Clypeomorus bifasciata, Lunella granulata and Japeuthria cingulata. C indicates the phylogenetic tree and the history of host utilization as revealed by ancestral trait reconstruction in P. conchopheria. *1 indicates Kunetsu population. 

Diversification through specialization in the algae covering molluscan shells

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貝殻に付着する藻類の特殊化を介した多様化

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