- 期刊
Sex Determination Studies in Two Species of Teleost Fish, Oreochromis Niloticus and Leporinus Elongatus
兩種硬骨魚Oreochromis Niloticus及Leporinus Elongatus性別決定之研究
摘要
經遺傳分析好幾種硬骨魚類的性別決定得以鑑定出其性染色體，然而有幾個性比偏離異配系統（heterogameticsystem）期望比的例子顯示出在性別決定的過程中，不是有其它性別決定的次要基因，就是會受到環境因子的影響。由於缺乏性染色體的遺傳標記因子，因此，使得性別決定次要基因的鑑定十分困難。又因為有若干種脊椎動物（包括一種為魚類）的性別決定具有溫度依存（temperature-dependent）的現象，所以，以溫度效應的研究來驗證環境影響性別的假說相形之下較為容易。為了瞭解魚類的性別決定，我們測試高溫對Oreochromisniloticus性比的影響，以及分離Leporinus elongatus性染色體上的分子標記。以36°C的水溫處理受精後一週的O. niloticus魚苗十天以上會明顯提高雄魚的比例，由後代分析證實牠們在遺傳組成上是雌性，但是後來發生了性轉變的現象。兩段位於L. elongatus的Z和W染色體上之非轉譯（non-coding）序列經由遺傳組扣除法（genomic subtraction）加以選殖（cloned），然後將這些序列與一個近緣種的遺傳組作雜結合（cross hybridize），結果並無特定的性別產生（sex-specific patterns）。一些L. elongatus的個體先根據其生殖系統及染色體作性別判定，然後以前述兩段序列進行DNA雜結合，結果顯示有三尾魚具非典型的W染色體，而有趣的是其中兩條是具有ZW核型（karyotype）的雄魚。因此，我們認為這些非典型的性染色體是Z和W染色體互換所致，同時這個魚種正由雌性異配轉型為雄性異配。
並列摘要
Genetic analyses of sex determination have identified sex chromosomes in many teleost fish species. However, there are several cases for which sex ratios do not fit perfectly with the expectations of heterogametic systems, suggesting the influence of either minor sex determining genes or environmental influences on the process of sex differentiation. The frequent absence of sex chromosome markers makes the identification of minor sex-determining genes very difficult. It is easier to test first the hypothesis of environmental sex determination' (ESD) by studying the temperature effect, since temperature-dependent sex determination has been demonstrated to occur in several vertebrate groups including 1 fish species. To contribute to a better understanding of fish sex determination, we have tested the effects of high temperatures on sex ratios of Oreochromis niloticus, and have attempted to isolate sex chromosome molecular markers in Leporinus elongatus. Treatments of O. niloticus fry at 36°C applied for 10 days and more, and starting 1 week after fertilization markedly increased the proportion of males, and progeny-testing these males confirmed that some of them are sex-reversed genetic females. Two non-coding sequences of L. elongatus Z and W chromosomes were cloned by genomic subtraction. They cross-hybridized with the genome of a close species without providing sex-specific patterns. A collection of L. elongatus individuals was subjected to gonadal and chromosomal sexing, and DNA hybridization with both sequences. These analyses revealed 3 individuals having atypical W chromosomes. Interestingly, 2 of these were males having a ZW karyotype. We assume that these atypical sex chromosome arise by exchanges between Z and W chromosomes, and that a transition between female and male heterogamety is underway in this species.