海草克隆性及其种群遗传效应

Clonality and its population genetic conseuqneces in seagrasses

海草是适应在海洋环境中生存和繁殖的单子叶植物,由于所处环境常存在潮汐、风暴等的干扰,海草形成了一系列适应特征,克隆性是其中突出的一个。所有的海草都具有水平根状茎,许多海草也具有垂直根状茎,在一些海草中,也观察到无性生殖(无融合生殖)。与克隆生长有关的参数(如节间长度、间隔子长度、分枝角度以及延伸速率和分枝率等)对于海草的克隆生长有着决定性影响,但繁育系统对克隆斑块大小也有较大影响。强烈的克隆性影响着海草的遗传变异。总体来看,海草种群内遗传多样性比陆生植物低,也低于另一类海洋高等植物-红树植物,利用DNA标记观察到的多样性高于等位酶标记。在一些海草植物种群中观察到较高的克隆多样性,但也有一些种群由单一基因型或少量基因型组成,其原因主要是由于奠基者效应和克隆生长。通常克隆植物中基因流有限,但是海草的克隆片段可能远距离扩散,从而提高种群间的基因流。就克隆生长对种群空间结构和交配系统的影响进行了综述。

Seagrasses are monocotyledonous species that are adapted to live and reproduce entirely in the marine environments, where they suffer from a variety of disturbances, such as tides, hurricanes. To adapt to these environments, seagrasses have evolved a series of characteristics, among which strong clonality is a prominent one. All the seagrasses have horizontal rhizomes, and many species have vertical rhizomes. In some seagrass species, clonal reproduction, via apomict, was also observed. We reviewed the parameters relating to clonal growth, such as internodal length, spacer length and branching angle as well as rhizome elongation and branching rates, of seagrass species. These parameters have determinant impacts on the expanding of seagrass patches. However, breeding system may also affect patch sizes. Strong clonality has essential influences on genetic variation of seagrasses. Generally, seagrass species have relatively lower within-population genetic variation than terrestrial plants or mangroves, another group of marine flowering plants, although DNA markers, such as microsatellites, have revealed much higher variation than allozymes did and higher than that of previous thought in seagrasses. High variation in clonal diversity was found in many seagrass populations, but single or few genotypes were also found in some populations due to founder effect and strong clonality. Although gene flow usually was thought to be restricted in clonal species, it was reported that clonal fragments may be transported to a long distance and therefore to promote gene flow among populations. Effects of clonality on within-population structuring and mating system were also reviewed.