Tocque K, Tinsley R C
School of Biological Sciences, Queen Mary and Westfield College, London University.
Parasitology. 1991 Apr;102 Pt 2:213-20. doi: 10.1017/s003118200006251x.
Pseudodiplorchis americanus accumulates its entire reproductive output in utero during hibernation of its host, Scaphiopus couchii. Infective larvae are released only during the toads' brief entry into water for breeding. In one host population studied prior to transmission, in S.E. Arizona, adult worms contained 0-326 infective stages; 19% of 178 worms produced 51% of the total larvae. Reproductive output was positively correlated with body length which was considered to reflect worm age; first-, second- and third-year worms produced means of 5, 43 and 96 larvae, respectively. Within second- and third-year cohorts, there was a density-dependent reduction in offspring numbers. Under controlled laboratory conditions, the mean rate of offspring accumulation during the first 6 months of development was significantly reduced only in burdens in excess of those found naturally (36-68 worms). In older parasites, a density-dependent reduction in the mean accumulation rate occurred in burdens of 20-32 worms, where 88% of parasites produced offspring at less than half the maximum rate. The asymmetry in individual reproductive output increased with competition and there were significant reductions in the median rates of offspring accumulation at smaller burdens (17-25 and 5-14 worms, in experimental and natural infections, respectively) than evident from decreases in the mean. Thus, individual variation is a more sensitive measure of competition than is the mean.
美洲伪双睾吸虫在其宿主科氏掘足蟾冬眠期间，于子宫内积累其全部生殖产出。感染性幼虫仅在蟾蜍短暂入水繁殖时释放。在亚利桑那州东南部传播前研究的一个宿主种群中，成虫体内含有0 - 326个感染阶段；178条虫中有19%产出了总幼虫数的51%。生殖产出与体长呈正相关，体长被认为反映虫龄；1龄、2龄和3龄虫分别平均产出5、43和96条幼虫。在2龄和3龄虫群体内，后代数量存在密度依赖性减少。在受控实验室条件下，仅在超过自然发现数量（36 - 68条虫）的虫负荷中，发育前6个月后代积累的平均速率才显著降低。在较老的寄生虫中，虫负荷为20 - 32条虫时，平均积累速率出现密度依赖性降低，其中88%的寄生虫产出后代的速率不到最大速率的一半。个体生殖产出的不对称性随竞争加剧而增加，在较小虫负荷（实验感染中为17 - 25条虫，自然感染中为5 - 14条虫）下，后代积累的中位数速率显著降低，比从平均值下降中明显看出的情况更显著。因此，个体变异是比平均值更敏感的竞争指标。
