Lewis S S, Klerks P L, Leberg P L
Department of Biology, University of Louisiana at Lafayette, P.O. Box 42451, Lafayette, LA 70504, USA.
Aquat Toxicol. 2001 May;52(3-4):205-16. doi: 10.1016/s0166-445x(00)00151-x.
Previous studies with Gambusia holbrooki have found associations of allozyme genotype with tolerance to metals, pesticides, heat, and salinity. To examine the generality of these relationships, we looked for similar associations of mercury and heat tolerance with allozyme genotype at the GPI-2, MDH-1 and MDH-2 loci in its sister species Gambusia affinis. This was done to assess if the loci themselves or closely linked loci were associated with mercury tolerance, because weaker linkage associations would be unlikely to persist across species boundaries. Moreover, the use of two very different types of stress allowed us to determine if the higher tolerance of particular allozyme genotypes is specific to a certain stress or reflects a higher tolerance to stress in general. Associations between genotype and tolerance to mercury and heat were determined in laboratory exposures of about 875 fish in each of two exposures where tolerance was measured as time-to-death, followed by electrophoresis on cellulose acetate gels. For none of the three loci did we find an association of genotype with tolerance to mercury. This contrasts with reports of such an association for GPI-2 and MDH-1 in G. holbrooki, so our results do not support the hypothesis that observed associations between allozyme genotypes and mercury tolerance are due to the allozymes or closely linked loci. However, our comparison was weakened by a scarcity of the GPI-2 genotypes reported to be mercury sensitive in G. holbrooki. Furthermore, rapid mortality in our mercury exposure may have affected the ability to detect genotypic differences in survival. The MDH-1 heterozygote showed higher tolerance to heat stress compared to homozygotes, although this difference was only significant for the most common homozygous genotype. No such relationship between MDH-1 and heat stress has been reported in G. holbrooki. We found no evidence that associations between allozyme genotype and tolerance are similar for different types of stresses, which could be an advantage for using allozymes as an indicator of exposure history to a stressor. Our study also showed that G. affinis sex and weight influence tolerance to mercury and heat.
先前对食蚊鱼（Gambusia holbrooki）的研究发现，等位酶基因型与对金属、农药、高温和盐度的耐受性之间存在关联。为了检验这些关系的普遍性，我们在其姊妹物种食蚊鱼（Gambusia affinis）的葡萄糖磷酸异构酶-2（GPI-2）、苹果酸脱氢酶-1（MDH-1）和苹果酸脱氢酶-2（MDH-2）基因座上，寻找汞耐受性和热耐受性与等位酶基因型之间的类似关联。这样做是为了评估这些基因座本身或紧密连锁的基因座是否与汞耐受性相关，因为较弱的连锁关联不太可能跨越物种界限持续存在。此外，使用两种截然不同的应激类型使我们能够确定特定等位酶基因型的较高耐受性是特定于某种应激，还是总体上反映了对压力的更高耐受性。在两次实验中，每次对约875条鱼进行实验室暴露，测定其对汞和热的耐受性（以死亡时间衡量），随后在醋酸纤维素凝胶上进行电泳，从而确定基因型与汞和热耐受性之间的关联。在这三个基因座中，我们均未发现基因型与汞耐受性之间存在关联。这与关于食蚊鱼（G. holbrooki）中GPI-2和MDH-1存在这种关联的报道形成对比，因此我们的结果不支持等位酶基因型与汞耐受性之间观察到的关联是由于等位酶或紧密连锁基因座的假设。然而，我们的比较因食蚊鱼（G. holbrooki）中据报道对汞敏感的GPI-2基因型数量稀少而受到削弱。此外，我们的汞暴露实验中鱼的快速死亡可能影响了检测存活基因型差异的能力。与纯合子相比，MDH-1杂合子对热应激表现出更高的耐受性，尽管这种差异仅在最常见的纯合基因型中显著。食蚊鱼（G. holbrooki）中尚未报道MDH-1与热应激之间存在这种关系。我们没有发现证据表明不同类型应激下，等位酶基因型与耐受性之间的关联是相似的，这对于将等位酶用作应激源暴露历史的指标可能是一个优势。我们的研究还表明，食蚊鱼（G. affinis）的性别和体重会影响其对汞和热的耐受性。
