Ebbert M A
Department of Biology, Yale University, New Haven, CT 06511.
Heredity (Edinb). 1995 Mar;74 ( Pt 3):227-40. doi: 10.1038/hdy.1995.36.
Male-lethal, maternally inherited spiroplasmas occur in four species of Drosophila, and persist in natural populations despite imperfect vertical transmission rates. In the field, larval crowding is thought to be sporadic, but occasionally intense. To determine whether crowding affects host persistence, I compared the population dynamics of infected females (hosts) under crowded conditions to those expected from data collected on uncrowded females. I estimated host fitness components and maternal transmission rates for individual females under uncrowded conditions in both the artificial host D. pseudoobscura (this paper) and the native host D. willistoni (previously reported). Spiroplasma infection had no effect on lifetime production of daughters in D. pseudoobscura; however, as with some D. willistoni lines, hosts may produce more of their daughters earlier in life than nonhosts. Because individual contributions to relative rates of increase calculated from these fitness data were similar for hosts and nonhosts, I expected hosts to persist in laboratory populations. Instead, three patterns were observed: rapid extinction of D. willistoni females infected with male-lethal spiroplasmas, slow decline or persistence of hosts (depending on initial frequency) in both D. pseudoobscura infected with male-lethal spiroplasmas, and D. willistoni infected with non-male-lethal spiroplasmas. Population dynamics, then, depend on host species and bacterial isolate. Fitness estimates change with host line in uncrowded D. willistoni, but host genetic background did not affect population dynamics. These and previously published results show that the interaction phenotype changes with host and parasite isolate, and that host fitness can be affected by crowding. Crowding in natural populations may therefore decrease host fitness but, in expanding populations, early reproduction in hosts may be to their advantage. Possible effects of seasonal fluctuations in population density on the fitness of infected Drosophila are discussed.
雄性致死、母系遗传的螺原体存在于四种果蝇中，尽管垂直传播率并不完美，但仍在自然种群中持续存在。在野外，幼虫拥挤被认为是零星发生的，但偶尔也会很严重。为了确定拥挤是否会影响宿主的持续存在，我将拥挤条件下受感染雌性（宿主）的种群动态与从未拥挤雌性收集的数据预期的动态进行了比较。我估计了人工宿主拟暗果蝇（本文）和本地宿主威氏果蝇（先前报道）在不拥挤条件下个体雌性的宿主适合度成分和母系传播率。螺原体感染对拟暗果蝇雌性后代的终生产量没有影响；然而，与一些威氏果蝇品系一样，宿主可能比非宿主在生命早期产生更多的雌性后代。由于根据这些适合度数据计算出的宿主和非宿主相对增长率的个体贡献相似，我预计宿主会在实验室种群中持续存在。相反，观察到了三种模式：感染雄性致死螺原体的威氏果蝇雌性迅速灭绝，感染雄性致死螺原体的拟暗果蝇和感染非雄性致死螺原体的威氏果蝇中宿主缓慢下降或持续存在（取决于初始频率）。因此，种群动态取决于宿主物种和细菌分离株。在不拥挤的威氏果蝇中，适合度估计随宿主品系而变化，但宿主遗传背景并不影响种群动态。这些以及先前发表的结果表明，相互作用表型随宿主和寄生虫分离株而变化，并且宿主适合度可能受到拥挤的影响。因此，自然种群中的拥挤可能会降低宿主适合度，但在种群扩张时，宿主的早期繁殖可能对它们有利。讨论了种群密度季节性波动对受感染果蝇适合度的可能影响。
