贡嘎山高低海拔上优势杨柳科植物性别比例与繁殖特性

Sex ratio patterns and reproductive characteristics of Salicaceae species at high and low altitudes on Gongga Mountain

雌雄异株植物的性比问题一直是进化生物学的研究热点之一.为揭示杨柳科植物不同性比格局的内在机理,从繁殖投入、种群密度和传粉效率等方面对贡嘎山两个海拔高度（2 000和2 600 m）上冬瓜杨和川滇柳的性别比例与繁殖投入进行对比研究.结果显示：（1）低海拔下冬瓜杨和川滇柳雄雌性比（M/F）接近1：1平衡;而在高海拔区域出现性比失衡,即杨树偏雄（M/F=2.36,P=0.008）,而柳树偏雌（M/F=0.62,P=0.033）.（2）冬瓜杨和川滇柳种群密度随海拔升高均明显下降,但川滇柳密度显著高于冬瓜杨,柱头接收花粉数量亦显著高于冬瓜杨.（3）枝条水平上,杨柳的总繁殖结构生物量（花序干重＋种子干重）在两个海拔上都为雌株高于雄株;相对繁殖投入在低海拔时雌株高于雄株,高海拔上冬瓜杨雌株高于雄株,而川滇柳由于叶片的补偿生长,包括更高的叶片生物量投入和光合能力,导致相对繁殖投入雌雄间无显著差异.冬瓜杨雌株繁殖投入显著高于雄株,雌株对高海拔更敏感,因而高海拔时偏雄性.川滇柳的繁殖投入在海拔间无差异,加之传粉效率较高,可能容易产生花粉管竞争,从而偏雌性.本研究发现较之低海拔,贡嘎山上杨柳科植物在高海拔上叶与繁殖结构生物量权衡关系改变容易导致性比失衡,未来需在分子水平上,结合性别决定基因、性染色体和表观遗传学等进一步深入研究.

The causes and consequences of variation in the sex ratios of organisms with separate sexes remain a central theme in evolutionary biology. The sex ratios of sympatric Populus sp. and Salix sp. vary according to the altitude and species; 1：1 equilibrium sex ratios are observed at mid-altitudes while the ratios are skewed at high altitudes, with Populus sp. having male-biased and Salix sp. having female-biased sex ratios. In this study, the reproductive investments, population density, and pollination efficiency of P. purdomii and S. magnifica were assessed at two different altitudes of 2 000 and 2 600 m on the Gongga Mountain, with the aim to provide new clues on the ecological and evolutionary aspects of the two species. Our results showed that the population densities significantly decreased from 2 000 to 2 600 m, being higher for S. magnificathan for P. purdomii at both the altitudes. The pollen received by the stigma was also significantly higher in Salix sp., due to its high population density, as well as due to the simultaneous pollination by the wind and insects, which may intensify the gametophytic competition to favor selective fertilization by the female-determining pollen tubes. Reproductive investments, including the biomass of the inflorescence and the seeds was higher in the females of Populus sp. at both the altitudes, whereas for Salix sp., the biomass was higher in the females at low altitudes, and was comparable to those at high altitudes. Under conditions of optimal temperature and precipitation, vigorous plant growth combined with relatively smaller reproductive investments was observed to contribute to the 1：1 equilibrium sex ratio, at an altitude of 2 000 m. At higher altitudes, the balance between the reproductive and vegetative growth was easily affected by more pronounced gender-related differences for reproduction, resulting in biased sex ratios, e.g.： Populus sp. exhibited male biasness and Salix sp. showed female biasness at high altitudes. Consequently, our results improve our understanding of the causes and consequences of sexual dimorphism in dioecious species. However, more information on the genetic or epigenetic basis of sex determination, including identification of the definite sex chromosomes and sex markers is urgently needed.