DIFFERENTIAL SURVIVAL OF SEXUAL AND ASEXUAL POECILIOPSIS DURING ENVIRONMENTAL-STRESS

According to the Frozen Niche-Variation model, coexisting clones of an asexual species can freeze and faithfully replicate ecologically rele vant genetic variability that segregates in the sexual ancestors. The present experiments with fish of the genus Poeciliopsis provide furthe r evidence in support of this model. Sexual and clonal forms of Poecil iopsis live in the desert streams of Sonora, Mexico, and are exposed t o environmental extremes ranging from flash floods to hot, desiccating , residual pools. We examined coexisting members of the monacha comple x to see whether the fish types differed with respect to survival duri ng stress and swimming endurance in an artificial flume. The two coexi sting clones of the triploid gynogenetic fish P. 2 monacha-lucida diff ered dramatically: clone MML/II had the best survival during heat and cold stress and the worst survival during hypoxic stress, whereas clon e MML/II had the best survival during hypoxic stress and the worst dur ing heat stress. Poeciliopsis monacha, the sexual species with which t hese clones coexist, had intermediate survival during heat and hypoxic stress and very poor swimming endurance in the flume. The physiologic al differences seen in this study are consistent with the Frozen Niche -Variation model and provide some insights into environmental factors that affect the distribution and abundance of these fish.