大山蝸牛屬及台灣山蝸牛屬之種化事件與山蝸牛科之系統發育學研究

Cyclophoridae phylogeny and the speciation events of Cyclophorus and Cyclotus taivanus ssp.

李彥錚(Yen-Chen Lee)

指導教授 ： 呂光洋 ； 巫文隆

國立臺灣師範大學/理學院/生命科學研究所/博士(2007年)

摘要

全世界約有300種山蝸牛，分屬於四個亞科，然而他們的系統發育學尚未明瞭。為了瞭解山蝸牛成員間的親源關係，本研究以COI及16S rRNA研究台灣及其鄰近地區10屬32種山蝸牛的親源關係，結果顯示Cyclophorus, Cyclotus, Leptopoma,及Platyrhaphe是單系群，傳統分類上的Japonia是複系群，其中有部分Japonia的成員應被劃分至新屬Pilosphaera。此外本研究發現二新種分別是 Pilosphaera yentoensis n. sp.及Japonia boonkioensis n. sp.。另外確定C. micron, C. ogaitoi, C. iota and C. taiwanicum 均屬於Cyathopoma屬而非Cyclotus屬。
台灣共有10屬29種山蝸牛，其中佔據相類似之生態棲位(niche)之Cyclophorus及Cyclotus 恰巧都有南北二型，分別分布於台灣南部及北部。為了研究這個現象本研究分別以Japonia 及Pterocyclus作為 Cyclophorus 及Cyclotus的外群來研究Cyclophorus 及Cyclotus的系統發生學關係。
台灣之Cyclophorus的COI及16S rRNA的基因樹都顯示明顯的地理結構，分布於台灣成環形結構，再台南附近二極端族群交會，鄰近族群間有限度的基因交流，二極端族群間幾乎無基因交流，推測Cyclophorus是一環種(ring species)。PLS分析顯示， Cyclophorus有稜殼型多發生於溫暖穩定的氣侯型態，此種殼形可能有利於在植被濃密的環境中爬行的適應有關。
在傳統的分類Cyclotus taivanus包含五個亞種，以COI及16S rRNA的分析顯示：Cyclotus的基因樹呈現明顯的雙演化支系，而將Cyclotus taivanus的所有亞種分成AB二群，A群為adamsi，因其與B群的遺傳距離已達山蝸牛科種階層的差異，因此應將adamsi提升為種。B群包含其他所有亞種。由PCA分析得知A(亦即C. adamsi)、B(包含C. t. dilatus, C. t. diminutus, C. t. peraffinis及C. t. taivanus)二群的棲地氣候不同，A為冬雨夏乾，B為冬乾夏雨，AB二群對環境的不同適應(天擇)，可能為促成二者累積遺傳變異的原因，因此造成種化。
Cyclophorus與Cyclotus是台灣中大型的山蝸牛，二者都廣泛分布於台灣，外殼都有許多變異，更佔據這相似的生態棲位。然而二者的演化歷史卻不盡相同，這說明即使是有相同的分布環境，受相同地質事件的影響，利用類似生態資源的二生物，即便是非常微小的差異，也會造成不同的演化歷史。

關鍵字

山蝸牛科 ； 大山蝸牛屬 ； 台灣山蝸牛屬 ； 種化 ； 系統發生學 ； 台灣

並列摘要

Cyclophoridae consists of four subfamilies and about 300 species currently arranged in 38 genera, occuping varies habitats, with great morphological diversity allover the world. However, the relationship among Cyclophoridae is thus far not clear. In order to investigate this, I sampled cyclophorid snails around Taiwan and its adjacent areas, and then sequenced part of the mitochondrial COI (cytochrome oxidase subunit I) and the 16S rRNA gene from 32 species of 10 cyclophorid genera to establish a phylogenetic tree of Cyclophoridae. Phylogenetic relationships based on mtDNA sequences suggest that Cyclophorus, Cyclotus, Leptopoma, and Platyrhaphe are monophyletic while the traditional genus Japonia is polyphyletic, and the previous J. zebra should be placed into a new genus Pilosphaera. In addition, Pilosphaera yentoensis n. sp. and Japonia boonkioensis n. sp. will also be described as new species. Members of Cyathopoma are tiny white cyclophorid snails occurring in East Asia, Madagascar and the Seychelles. Phylogenetic relationships of Cyathopoma are uncertain. Combined with COI and radular data, I conclude that Cyathopoma and Cyclotus are only distantly related. Cyathopoma iota has been considered to be a controversial member of this group. Through molecular and radular data, I found C. iota to be closer to C. taiwanicum than to C. micron, and concluded that C. micron, C. ogaitoi, C. iota and C. taiwanicum all belong to Cyathopoma.
There are 10 genera and 29 cyclophorid species in Taiwan. Among them, the most interesting taxa are Cyclophorus and Cyclotus, both sharing similar ecological niches and representing by a north and south form in morphology. In order to clarify their relationship, I have to find out their sister group as out groups to compare with the members among Cyclophorus and Cyclotus. The gene trees of Cyclophoridae indicate that Japonia and Pterocyclus are sister group of Cyclophorus and Cyclotus, respectively. The former two will be used as the out groups of Cyclophorus and Cyclotus in their phylogenetic studies.
Both COI and 16S rRNA gene trees of Taiwan Cyclophorus show prominent geographic structure. The Mantel test showed significant positive correlation between fixation index (FST) and cumulative geographic anti-clockwise distance (origin in the region around Tainan, anti-clockwise pass through south cape, Taidung, Hualian, Iran, Taipei, Taichung and meet the original populations in Jia-yi). There are finite gene flew between adjacent populations. And there are series of clines around the Central Range. Cyclophorus of Taiwan is a proposed “ring species”. In the morphology and environmental variables correlation study, I found the currently shell morphology may be caused by the adaptations of recent long term climate.
In traditional classification, Cyclotus taivanus consists of five subspecies, with clear morphological diversity. The molecular phylogenetic relationships of this group have never been discussed before. I sequenced part of the mitochondrial COI (cytochrome oxidase subunit I) and the 16S rRNA gene from 27 sampling sites. I also measured 9 shell traits to investigate the relationships between C. taivanus ssp. Even though the morphology PCA revealed a more or less continuous distribution of individuals in morph-space, the two highly divergent haplotype clades in COI and 16S rRNA analysis indicated the presence of two independently evolving lineages. The sequence divergence between two clades was almost as high as between other Cyclophoridae species. Therefore C. adamsi should be a considered valid species. For the environmental analysis, temperature may be a limiting element to the distribution of C. adamsi and C. taivanus group. The ecological divergence probably is the ruling force of speciation in my case. The PLS analysis results indicate, that phenotypic plasticity may be a key element of variable shell in C. taivanus group. The ecological divergence probably appears rule of speciation in C. taivanus ssp. case. The speciation process may be incomplete among C. t. dilatus, C. t. diminutus, C. t. peraffinis, and C. t. taivanus, and the adaptation of climatic pressure continue being a rule of speciation process.
This study provides an opportunity to understand that no matter how similar two taxonomic groups are, occupying similar niche, undergoing the same geology history, with morphological adaptation to the same long term climate, they may have different speciation model.

並列關鍵字

Cyclophoridae ； Cyclophorus ； Cyclotus ； speciation ； phylogeny ； Taiwan

參考文獻

Akcakaya, H.R. & Baur, B. 1996. Effects of population subdivision and catastrophes on the persistence of a land snail metapopulation. Oecologia, 105: 475–483.

Barrett, R.D.H. & Hebert, P.D.N. 2005. Identifying spiders through DNA barcoding. Canadian Journal of Zoology, 83: 481–489.

Benson, W.H. 1856. Characters of 17 new forms of the Cyclostomacea from the British province of Burmah collected by W. Theobald jun., Esq. Annals and Magazine of Natural History London, 2(17): 225-233.

Chiba, S. 1999. Accelerated evolution of land snails Mandarina in the oceanic Bonin Islands: evidence from mitochondrial DNA sequences. Evolution, 53(2): 460–471.

Cook, L.M. & Pettitt, C.W.A. 1979. Shell form in Discula polymorpha. Journal of Molluscan Studies, 45: 45–51.