性進(jìn)化在自然界物種的進(jìn)化過程中占有重要地位，但關(guān)于這一話題仍有諸多未解之謎,。一些物種,，如細(xì)菌、部分植物和爬行動(dòng)物,，采取無性生殖方式繁育后代,，單個(gè)個(gè)體即能夠完成生殖；而有性生殖的物種,，必須雌雄交配方能“生兒育女”,。如此看來，無性生殖似乎效率更高,，但在當(dāng)今自然界中,，占據(jù)統(tǒng)治地位的卻是有性生殖。
瑞士聯(lián)邦水產(chǎn)科技研究所教授朱卡·約凱拉,、美國華盛頓大學(xué)副教授馬克·迪布達(dá)爾,、印第安納大學(xué)教授庫爾蒂斯·利文斯提出的有關(guān)物種性進(jìn)化的“寄生蟲假說”或許可以回答這一問題?？茖W(xué)家們在《美國博物學(xué)家》雜志7月刊上發(fā)表文章指出,，自然界各物種在性方面的進(jìn)化可能是應(yīng)對(duì)寄生蟲的一種防御手段。
“寄生蟲假說”認(rèn)為,，寄生蟲可以阻止無性生殖的有機(jī)體大量繁育,。當(dāng)無性生殖的物種繁殖時(shí)，個(gè)體會(huì)自我克隆,，分毫不差地將自己的基因完全復(fù)制,。由于各個(gè)體擁有完全相同的基因，它們也擁有針對(duì)寄生蟲的完全相同的弱點(diǎn),。如果某種能利用這些弱點(diǎn)的寄生蟲迅速興起,，整個(gè)群體就可能被摧毀。另一方面,，有性生殖物種的后代自身的基因都是獨(dú)特的,，因此，某些寄生蟲能夠破壞該物種的一部分,，卻無法摧毀整個(gè)物種,。理論上講，這可以使有性生殖群體保持穩(wěn)定,，而無性生殖物種受制于某些寄生蟲時(shí)就可能面臨滅頂之災(zāi),。
這一假說建立在數(shù)學(xué)模型基礎(chǔ)上，不過科學(xué)家在自然界中也的確觀測到了類似例子,。約凱拉等人自1994年起,，對(duì)一種小型水生蝸牛——新西蘭泥蝸的幾個(gè)種群進(jìn)行了為期10年的觀測,。
新西蘭泥蝸具有有性生殖和無性生殖兩個(gè)版本?？茖W(xué)家們分別監(jiān)控了這兩個(gè)版本的種群數(shù)量變化以及它們受寄生蟲感染的情況,?？茖W(xué)家發(fā)現(xiàn),，在初始階段，無性生殖的新西蘭泥蝸非常繁盛,，但隨著時(shí)間的推移,，這部分泥蝸越來越容易受到寄生蟲感染，它們的數(shù)量也急劇減少,，其中一些亞型甚至完全消失,。有性生殖的新西蘭泥蝸種群數(shù)量則非常穩(wěn)定。研究人員認(rèn)為,，這是“寄生蟲假說”的實(shí)例,。
“研究表明，在寄生蟲豐富的環(huán)境中,，有性生殖可以賦予物種進(jìn)化優(yōu)勢,，”約凱拉表示。（生物谷Bioon.com）
生物谷推薦原始出處：
Am Nat 2009. Vol. 174, pp. S43–S53 DOI: 10.1086/599080
The Maintenance of Sex, Clonal Dynamics, and Host‐Parasite Coevolution in a Mixed Population of Sexual and Asexual Snails
Jukka Jokela,1,* Mark F. Dybdahl,2 and Curtis M. Lively3
1. EAWAG, Swiss Federal Institute of Aquatic Science and Technology, überlandstrasse 133, 8600 Dübendorf, Switzerland, and ETH‐\Zürich, Swiss Federal Institute of Technology Zürich, Institute of Integrative Biology, 8092 Zürich, Switzerland;
2. School of Biological Sciences, Washington State University, Pullman, Washington 99164;
3. Department of Biology, Indiana University, Bloomington, Indiana 47405
Sexual populations should be vulnerable to invasion and replacement by ecologically similar asexual females because asexual lineages have higher per capita growth rates. However, as asexual genotypes become common, they may also become disproportionately infected by parasites. The Red Queen hypothesis postulates that high infection rates in the common asexual clones could periodically favor the genetically diverse sexual individuals and promote the short‐ erm coexistence of sexual and asexual populations. Testing this idea requires comparison of competing sexual and asexual lineages that are attacked by natural parasites. To date no such data have been available. Here, we report on long‐ erm dynamics and parasite coevolution in a “mixed” (sexual and asexual) population of snails (Potamopyrgus antipodarum). We found that, within 7–10 years, the most common clones were almost completely replaced by initially rare clones in two different habitats, while sexuals persisted throughout the study period. The common clones, which were initially more resistant to infection, also became more susceptible to infection by sympatric (but not allopatric) parasites over the course of the study. These results are consistent with the Red Queen hypothesis and show that the coevolutionary dynamics predicted by the theory may also favor sexual reproduction in natural populations.
