寄主隔離對四紋豆象棄卵決策之效應

Abstract

在寄主隔離的條件下，四紋豆象雌蟲將卵產在器壁等不適當處之行為稱之為棄卵。由於四紋豆象羽化後即不取食，在能量有限的條件下，此舉既耗費能量又無法產生有效子代，故以往被認定為礙於生理限制所產生的不適應行為。本研究發現，在寄主資源豐富時，產卵數呈連續分布。然而，在寄主隔離的條件下，族群內棄卵數卻呈不連續之兩群，即棄卵雌蟲與非棄卵雌蟲。透過比較該兩群雌蟲之生活史特質，並解剖其卵巢比較腹內成熟卵數，排除生理限制(如：雌蟲體重、卵成熟、載卵能力)導致該兩群差異之可能。根據寄主經驗處理與行為觀察的結果，我們發現棄卵表現受到雌蟲經驗的影響。又錄影結果顯示雌蟲棄卵前必有評估之過程，即使在沒有寄主時，足跡費洛蒙仍可作為產卵選擇的依據，雌蟲尋找足跡費洛蒙濃度低的位置棄卵，因而提出「決策假說」用以完整解釋棄卵行為之分歧，即非棄卵雌蟲並非無能力產卵之不正常雌蟲，停止產卵乃雌蟲之決策；棄卵雌蟲並非無能力承受載卵壓力，產卵在培養皿上乃雌蟲評估後之決策。

Foraging theory shows the delicate adaptation of animals to utilize their resources for oviposition. It is puzzling that some insect species dump their eggs on unsuitable places. This egg-dumping behavior, in previous studies, was generally considered as a maladaptive behavior under physiological constraints. In this study, physiological-constraint hypothesis of egg-dumping behavior in Callosobruchus maculatus was tested. It was revealed that a continuous distribution of egg-numbers was differentiated into two groups, dumpers and non-dumpers when the host was deprived. Different strategies may be adopted by females of these two groups to deal with host deprivation. By dissecting their ovaries and comparing the life history traits of the two groups, we falsified the hypothesis that physiological conditions (i.e., female weight, egg maturation, egg-storing ability) result in the various egg-dumping behaviors of females. Herein we integrated a new hypothesis in egg-laying decision view. Under a condition of host deprivation, non-dumpers were able to produce and lay eggs but reject unsuitable host. Dumpers compared the trail left on Petri dish with previous host experience and hold mature eggs in their body till they found an acceptable oviposition site. It thus broadened the views of egg-dumping from ovaries to trail pheromone, i.e., trial pheromone act as a cue for egg-dumping decision when host were deprived. Further, the strategies of egg-dumping behavior were characterized as non-dumpers waiting for a higher-quality host, and dumpers continuing to search for a better host even in a deprived situation.