海鞘可能并不漂亮,，但這種其貌不揚的海洋生物卻引起了科學家的濃厚興趣！海鞘可能受到許多物種的羨慕嫉妒,，它從不必擔心“避孕節(jié)育”或試管受精等問題,，便能夠有效地控制好生育繁殖。
這項由澳大利亞昆士蘭州大學進行的研究報告發(fā)表在8月26日美國權威專業(yè)期刊《美國國家科學家學報》上,，研究人員揭示海鞘這種海洋生物天然具有控制生殖循環(huán)的能力,，可依據需要或多或少地進行有節(jié)制繁殖。
該項研究的負責人是澳大利亞昆士蘭州大學綜合生物學院博士生研究員安吉拉－克林,，她指出,，海鞘能夠依據海洋環(huán)境中性別比例狀況,，“適時定制”自己的生殖細胞,。比如：當海洋環(huán)境中存在著大量的雄性海鞘，它們試圖競爭與雌性的卵細胞結合繁殖生育,，因此雄性將生產出更大,、更具競爭性的精子，便于存活更長的時間,。同樣地,，當雌性海鞘探測到過多的雄性競爭交配結合卵細胞時（過多的精子將殺死一些生物體的卵細胞），雌性海鞘將生產出更小的卵細胞,，很精子很難探測到,。
克林說，“如果為了避免在高競爭環(huán)境中失控,，它們必須更具競爭力地繁殖生育,，從而提高繁殖高成功率。”據了解,，在莫爾頓海灣的實驗地點的研究過程中,，他們發(fā)現(xiàn)事實上海鞘并未移動離開該海域。
在該實驗中,，大型的海鞘放置在狹窄海域進行深入研究分析,，克林說,，“我們分別將1只（低密度）或15只（高密度）海鞘放在實驗籠中進行1個月的觀測分析。這項研究告訴我們關于海鞘性特征的部分秘密,。當海鞘精子變得很小時,，卵細胞就會發(fā)育得更大，因此繁殖生育的數量會很少,。”（生物谷Bioon.com）
生物谷推薦原始出處：
PNAS Published online before print August 29, 2008, doi: 10.1073/pnas.0806590105
Gamete plasticity in a broadcast spawning marine invertebrate
Angela J. Crean* and Dustin J. Marshall
Sperm competition has classically been thought to maintain anisogamy (large eggs and smaller sperm) because males are thought to maximize their chance of winning fertilizations by trading sperm size for number. More recently it has been recognized that sperm quality (e.g., size, velocity) can also influence sperm competition, although studies have yielded conflicting results. Because sex evolved in the sea, debate has continued over the role of sperm competition and sperm environment in determining both sperm and egg size in externally fertilizing broadcast spawners. Remarkably, however, there have been no direct tests of whether broadcast spawners change the traits of their gametes depending on the likelihood of sperm competition. We manipulated the density (and thus, sperm environment) of a broadcast spawning ascidian (Styela plicata) in the field and then determined whether the phenotype of eggs and sperm changed. We found that sperm from adults kept at high density were larger and more motile than sperm from low-density adults. In vitro fertilizations revealed that sperm from high-density adults also lived longer and induced less polyspermy. Adult density also affected egg traits: eggs from high-density adults were smaller targets for sperm overall but produced larger ovicells than eggs from low-density adults. This suggests that broadcast spawning mothers balance (potentially conflicting) pre- and postzygotic selection pressures on egg size. Overall, our results suggest that sperm competition does not represent a strong force maintaining anisogamy in broadcast spawners. Instead, sperm limitation seems to select for large eggs and smaller, more numerous sperm.
