雌雄同花植物的性分配

Sex allocation in hermaphroditic plants

性分配理论假定雌雄功能之间存在着trade-off,对一种性别的投入增多必然会减少对另一性别的投入。雌雄功能投入的适合度曲线的形状决定了哪种繁育系统是进化稳定的。因此,性分配理论可以解释植物繁育系统的进化,尤其被认为是雌雄异株进化的选择机制之一。目前的实验研究分别在物种间、种群间、个体间及花间四个层次上进行:自交率的程度对物种和种群的性分配都有影响;虫媒和风媒植物的性分配是个体大小依赖的;而且花序内花的性分配模式受昆虫访花行为的影响。相对于理论,性分配的实验研究明显滞后,随着分子标记技术的普及,性分配理论将会获得更大的发展。繁殖分配需要进一步与性分配理论结合,尤其在空间尺度上资源分配与繁育系统变化的研究是很有意义的。

The theory of sex allocation assumes that there is a trade-off between the sexes, i.e. increasing investment to one gender will reduce the investment to another. The classical model of sex allocation for outcrossing species shows that male fertility （m） and female fertility （f） are considered power functions of the proportion of resources put into male function （r） ：m kmr^b, f=k1 （1- r）^c. Exponents （b and c）〈 1 generate a function with diminishing returns, meaning the ESS is hermaphroditism, whereas exponents （b and c）〉1 generate a function with accelerating returns and an ESS of dioecy. Sex allocation theory has been used to explain not only the evolution of breeding systems such as dioecy, but also to model allocation in partial selfers Most studies have focused on the four levels of flower, individual, population and species. Because plants are sessile, unlike animals, they are dependent upon pollen vectors for fertilization of ovules. This has contributed to the evolution of complicated reproductive modes and mating strategies. The different mating systems （such as selfing, cloning and outcrossing） have great effects not only on the fitness of individuals and the genetic structure of populations, but also on sex allocation within flowers. For example, the assumption that allocation to male function, including the proportion of resources allocated to attractive structures, declines with increasing selfing rate has been proven by many experiments. The effect of selfing rates on sex allocation can also be applied to comparisons among populations （or genotypes） within species, where the genotype with higher selfing rate allocates less to pollen production. Selfing can occur through various modes, however, including autogamous, abiotic geitonogamy and biotic geitonogamy. Studies have shown the ESS strategy is female-biased allocation for all three modes above. In poor environmental conditions （e. g. lower temperature）, pollinator abundance may decrease and selfing may increase, leading to a similar female-biased ESS. However, at higher elevation, an alternative strategy may be increased allocation to attractive structures （male）. The adaptive consequences of allocation responses to the environment need more study, especially in the context of mating system evolution. Recently, studies of sex allocation have focused on variation among individuals within populations. If fitness gain through male and female functions is differentially influenced by size or status, it is expected that organisms will adjust sex allocation according to size. Several theories have been synthesized to determine the degree and direction of size-dependency of sex allocation, including local mate or resource competition, the pollination environment, and resource costs associated with each sex function, Recent studies have illustrated the correlation of sex allocation and size in insect-pollinated plants, i.e. female- biased allocation of larger individuals, although wind-pollinated plants may allocate more to either function with increasing size. In general, larger inflorescence will promote geitonogamy and pollen discounting, which affects the ESS sex allocation as well. Within individuals with complex inflorescences, floral sex allocation at different positions within the inflorescence may change with the directionalily of visits by pollinating animals. In addition, variation in the amount of resources available to flowers at different positions within inflorescences may lead to variation among flowers in pollen and seed production. In contrast to the development of theories, experimental study of sex allocation has lagged. With the development of molecular markers, predictions of sex allocation theory, in particular the trade-off between the sexes can be more conveniently tested. Further research is necessary; especially the investigation of the evolution of resource allocation and breeding systems within a spatial context （including the effect of elevation on pollen limitation and sex allocation）. This work would be significant for our understanding of the evolution of life history traits in plants.