种子大小变异的进化生态学研究现状与展望

The present situation and prospect of studies on evolutionary ecology of seed size variation

种子大小是植物生活史的核心特征之一。从 4个方面综述种子大小变异进化生态学研究 :1理论及研究方法 ;2环境、系统发育和母系影响 ;3进化机制 ;4生态学和进化意义。分析目前研究中存在的问题 ,并提出今后的研究方向。

Seed size is one of the central features of a plant species’ life history. We comment on the following four aspects of research on the evolutionary ecology: (1) theory Seed size within species is usually regarded as relatively constant. A theoretical reason for expecting this is that one level of investment will have the highest fitness to investment ratio if the returns in offspring fitness on resources invested are diminishing (concave-downward). Many recent articles have emphasized that seed size does vary within species. There are two theoretical reasons: the first is that seed size variability is a consequence of variable genetic quality among offspring, because offspring genetic variation will affect the optimal pattern of maternal allocation; the second is that positive correlations between size and number may occur among individuals due to individual variation in resources, which implies some sort of density-dependent interactions among offspring. Within a plant community seed mass often varies over 3 to 5 orders of magnitude. However, this variation is not accounted for by reference to the preceding theories. In game-theoretical evolution of seed mass in mutli-species ecological models, when seed size forms part of a co-existence mechanism there may be no single seed mass ESS and instead, there may be an evolutionarily stable coalition of several species each with a different seed mass. (2) Environmental, phylogenetic and maternal factors Seed size variation may be determined not only by the genotype and environment of the offspring (seed) but also by maternal effects within species. The former include the chromosome from each parent and position on the maternal plant, and the latter includes cytoplasmic genetic, endosperm nuclear, and the environment or genotype of the maternal parent. The maternal parent′s environment involves factors such as water, temperature, nutrients and so on. The effect of genotype of the maternal parent on seed mass represent the tissues surrounding the developing embryo and the wall of the ovary, which eventually form the seed coat, fruit and accessory seed structures. Seed size variation is associated with environment, other plant traits and phylogeny among species. Environments involve light, water, and altitude. Other plant traits include longevity, growth form and dispersal. Phylogeny represents the evolutionary history of the species. (3) ecological significance Studies have shown an effect of seed size within species on germination characteristics, on seedling size, on relative growth rate, and on adult plant size and competitive ability. Seed size variation among species is associated with seedling size, emergence depth from soil or litter, shoot root ratio, relative growth rate, and avoiding predation. (4) evolutionary significance It has been suggested that within-plant variance in seed size represents an adaptation,with the variance evolving in response to variable environments. Hence, variably-sized seeds will result in a more homogeneous seed shadow. Because of genotype interactions with environment, neither approach can determine whether seed size variation is due to genetic differentiation rather than to phenotypic plasticity. So it is problematic whether seed size variation within species is adaptive. As to seed size variation among species, one view attributes this to stabilizing selection, another to developmental canalization. It is not possible to distinguish between these two hypotheses at present. Mechanisms of seed size variation is still poorly understood whether within species or among species. Future research will probably spend more effort determining the effect of seed genotype, maternal genotype and environment on seed size variation within species and quantifying the extent to which seed size variation is correlated with phylogeny (taxonomic affiliation). Dispersal and establishment biology will obviously be important under global change. Accordingly, relations between seed size and other plant traits have now become a significant applied issu