Lexer Christian, Welch Mark E, Raymond Olivier, Rieseberg Loren H
Indiana University, Department of Biology, Jordan Hall 325, Bloomington, Indiana 47405, USA.
Evolution. 2003 Sep;57(9):1989-2000. doi: 10.1111/j.0014-3820.2003.tb00379.x.
Diploid hybrid speciation in plants is often accompanied by rapid ecological divergence between incipient neospecies and their parental taxa. One plausible means by which novel adaptation in hybrid lineages may arise is transgressive segregation, that is, the generation of extreme phenotypes that exceed those of the parental lines. Early generation (BC2) hybrids between two wild, annual sunflowers, Helianthus annuus and Helianthus petiolaris, were used to study directional selection on transgressive characters associated with the origin of Helianthus paradoxus, a diploid hybrid species adapted to extremely saline marshes. The BC2 plants descended from a single F1 hybrid backcrossed toward H. petiolaris. The strength of selection on candidate adaptive traits in the interspecific BC2 was measured in natural H. paradoxus salt marsh habitat. Positive directional selection was detected for leaf succulence and Ca uptake, two traits that are known to be important in salt stress response in plants. Strong negative directional selection operated on uptake of Na and correlated elements. A significant decrease in trait correlations over time was observed in the BC2 population for Na and Ca content, suggesting an adaptive role for increased Ca uptake coupled with increased net exclusion of Na from leaves. Patterns of directional selection in BC2 hybrids were concordant with character expression in the natural hybrid species, H. paradoxus, transplanted into the wild. Moreover, the necessary variation for generating the H. paradoxus phenotype existed only in the BC2 population, but not in samples of the two parental species, H. annuus and H. petiolaris. These results are consistent with the hypothesis that transgressive segregation of elemental uptake and leaf succulence contributed to the origin of salt adaptation in the diploid hybrid species H. paradoxus.
植物中的二倍体杂交物种形成通常伴随着初始新物种与其亲本类群之间的快速生态分化。杂种谱系中可能产生新适应性的一种合理方式是超亲分离，即产生超过亲本系表型的极端表型。两个野生一年生向日葵物种，即向日葵（Helianthus annuus）和叶柄向日葵（Helianthus petiolaris）之间的早代（BC2）杂种，被用于研究与适应极端盐碱沼泽的二倍体杂交物种——奇异向日葵（Helianthus paradoxus）起源相关的超亲性状的定向选择。BC2植株源自单个F1杂种与叶柄向日葵的回交。在自然的奇异向日葵盐沼栖息地中，测量了种间BC2中候选适应性状的选择强度。检测到叶片肉质化和钙吸收存在正向定向选择，这两个性状在植物盐胁迫响应中已知是重要的。对钠和相关元素的吸收存在强烈的负向定向选择。在BC2群体中，随着时间的推移，钠和钙含量的性状相关性显著降低，这表明增加钙吸收并同时增加叶片对钠的净排斥具有适应性作用。BC2杂种中的定向选择模式与移植到野外的自然杂交物种奇异向日葵中的性状表达一致。此外，产生奇异向日葵表型所需的变异仅存在于BC2群体中，而在两个亲本物种向日葵和叶柄向日葵的样本中不存在。这些结果与以下假设一致，即元素吸收和叶片肉质化的超亲分离促成了二倍体杂交物种奇异向日葵的盐适应性起源。
