Poulin Robert
Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand.
Evolution. 1995 Apr;49(2):325-336. doi: 10.1111/j.1558-5646.1995.tb02245.x.
The evolution of reproductive strategies and the trade-off between number and size of eggs were investigated in a comparative analysis of free-living and parasitic copepods. Data from 1038 copepod species were used to obtain family averages for 105 families; the phylogenetic relationships among these families include 94 branching events or 94 independent contrasts on which the analysis was based. Transition from a free-living existence to parasitism on invertebrates resulted in small increases in body size. Transition from parasitism on invertebrates to parasitism on fish was associated with greater increases in body size. After controlling for body size, a switch to fish hosts resulted in an increase in the number of eggs produced and a reduction in egg size. Among all contrasts, there was a negative relationship between changes in relative clutch size and changes in relative egg size, suggesting the existence of a trade-off between egg size and numbers. However, opposite changes in these measures of clutch size and egg size were not quite more frequent than expected by chance, therefore indicating that investments into egg numbers are not necessarily made at the expense of egg size, and vice versa. Latitude affected copepod body size, clutch size, and egg size, whereas the effects of freshwater colonization or size of the fish host were not significant. Comparative analyses at either the genus or species levels within given taxa of copepods parasitic on fish provided limited support for a trade-off between clutch size and egg size, but were hampered by the small number of independent phylogenetic contrasts available. From the family-level comparative analysis, it appears that the evolutionary transition from a free life to parasitism on invertebrates, and the transition from parasitism on invertebrates to parasitism on fish, have led to changes in life-history traits in response to the different selective pressures associated with the different modes of life.
在对自由生活和寄生性桡足类动物的比较分析中，研究了生殖策略的演变以及卵的数量和大小之间的权衡。利用来自1038种桡足类动物的数据，得出了105个科的科平均值；这些科之间的系统发育关系包括94个分支事件或94个独立对比，分析就是基于这些进行的。从自由生活向寄生在无脊椎动物上的转变导致体型略有增加。从寄生在无脊椎动物向寄生在鱼类上的转变与体型的更大增加有关。在控制体型后，转而寄生在鱼类宿主上会导致产卵数量增加和卵大小减小。在所有对比中，相对产卵量的变化与相对卵大小的变化之间呈负相关，表明卵大小和数量之间存在权衡。然而，这些产卵量和卵大小指标的相反变化并不比偶然预期的更频繁，因此表明对卵数量的投入不一定是以牺牲卵大小为代价，反之亦然。纬度影响桡足类动物的体型、产卵量和卵大小，而淡水定殖或鱼类宿主大小的影响不显著。对寄生在鱼类上的桡足类动物特定分类单元内的属或物种水平进行的比较分析，为产卵量和卵大小之间的权衡提供了有限的支持，但受到可用的独立系统发育对比数量较少的阻碍。从科级比较分析来看，从自由生活向寄生在无脊椎动物上的进化转变，以及从寄生在无脊椎动物向寄生在鱼类上的转变，似乎导致了生活史特征的变化，以应对与不同生活方式相关的不同选择压力。
