探討海洋底棲型毛顎動物種化之機制

Abstract

毛顎動物，又稱為「箭蟲」，是一群掠食性的海洋無脊椎動物，在分類上屬於毛顎動物門 (Chaetognatha) 。此門的物種不管在演化支序的界定或系統分類上都困擾著科學家相當多年。在過去，毛顎動物的物種劃分都僅依靠形態上的分類，直到分子遺傳學的技術逐漸發展，有更多基因上分化的族群被發現，甚至可能存在未被定義的隱蔽種。然而，過去的研究並沒有探討物種間的生殖隔離，使得我們並不清楚這些基因分化的族群或隱蔽種是否存在生殖上的隔離屏障。而底棲型的毛顎動物（鋤蟲科 Spadellidae）因其特殊的生長習性，例如易於飼養和可觀察的交配行為，使得底棲型毛顎動物成為適合探討生殖隔離的模式物種。因此在本研究中，我們欲探討一底棲型毛顎動物（鋤蟲屬物種 Spadella sp.）的基因多樣性和其交配的偏好行為，來了解其生殖隔離和種化的過程。實驗用的底棲型毛顎動物 (Spadella sp.) 皆採自澎湖群島和東沙島。我們在澎湖橫礁與嵵裡的採集點發現具有些微形態差異的 Spadella sp. 個體，而根據研究結果指出，來自澎湖嵵裡的 Spadella sp. 族群具有相當程度的選型交配行為，與其形態上的差異一致。然而在親緣關係的分析上，分化的程度與方向似乎與地理分布較為相關，而非形態上的差異。這樣的結果顯示出位於澎湖的 Spadella sp. 族群可能存在雜交和基因漸滲的情況。The chaetognaths, or so-called"arrow worms", are predatory marine invertebrates belonging to an animal phylum, Chaetognatha. The taxonomy and phylogeny of this phylum had puzzled scientists for many decades. In the past, chaetognath species were solely identified by morphological characters. With the recent advancement of molecular genetic techniques, genetically distinct populations or even cryptic species were reported frequently. However, none of the previous studies had examined the reproductive isolation of the species.Therefore, whether those cryptic species represent reproductively isolated species or not is unclear. Benthic chaetognaths (the family Spadellidae) are a good model to study reproductive isolation because they are easy to culture and mate under captive conditions. In the present study, we examined the genetic diversity and the mating preference behavior of a benthic chaetognath species (Spadella sp.) to understand their reproductive isolation and speciation processes. Individuals of Spadella sp. were collected from Dongsha and Penghu Islands. We found individuals with a small morphological difference in Hengjiao and Shihli, Penghu. The results showed that the benthic chaetognaths collected in Shihli, to some degree, have assortative mating behaviors which are concordant with the morphological characters. However, the phylogenetic relationship seemed to agree with geographical distribution, not morphology. These results suggest that the hybridization and introgression may have happened in the Penghu population.