The Compton Herbarium, Kirstenbosch Research Centre, South African National Biodiversity Institute, Private Bag X7, Newlands, Cape Town 7735, South Africa.
Am J Bot. 2011 Jul;98(7):1113-27. doi: 10.3732/ajb.1000457. Epub 2011 Jun 23.
Evolutionary significance of the Compositae capitulum and variation in its structure is poorly understood, although it may permit flexibility in sexual expression. Optimal sex ratio differs with life-history and reproductive strategy. We explore how the genus Ifloga and related members of southern African Gnaphalieae achieved different sex ratios, and the associations of these ratios with annual and perennial life history.
Sex allocation was measured using the male to female ratio (M/F), a novel approximator of the pollen to ovule ratio (P/O). Life-history (annuality/perenniality), capitulum structure, capitular sexual system, and M/F were reconstructed on time-proportional phylogenies. Trait associations were examined using phylogenetically independent contrasts (PICs).
Annual taxa have strongly female-biased capitula, as measured by M/F, and either gynomonoecious or monoecious sexual systems, while perennials have equal or male-biased capitula that are hermaphroditic or monoecious. These results are largely supported by PIC analysis. Different sexual systems afford differing flexibility in sex allocation, with hermaphrodites having the least, and monoecious taxa the greatest, range in M/F. Within Ifloga, the anomalous capitulum evolved in an annual, gynomonoecious ancestor, followed by two independent gains of monoecy. Two subsequent gains of perenniality occurred within a monoecious sublineage.
Different life histories have divergent sex allocation optima and are strongly associated with different sexual systems in gnaphalioid daisies. An anomalous capitulum structure in Ifloga may have facilitated the evolution of monoecy, which in turn may be linked to the evolution of life-history diversity in the genus.
尽管头状花序的进化意义以及其结构的变异尚不清楚，但它可能会使有性表达具有灵活性。最佳性别比例因生活史和生殖策略而异。我们探讨了 Ifloga 属和南部非洲 Gnaphalieae 的相关属如何实现不同的性别比例，以及这些比例与一年生和多年生生活史的关联。
使用雄性与雌性的比例（M/F）来衡量性别分配，这是花粉与胚珠比（P/O）的新近似值。使用时间比例系统发育重建生活史（一年生/多年生）、头状花序结构、头状花序性系统和 M/F。使用系统发育独立对比（PIC）分析来检验性状关联。
一年生类群的头状花序具有强烈的雌性偏向，这可以通过 M/F 来衡量，并且具有雌雄同株或雌雄异株的性系统，而多年生类群的头状花序具有相等或雄性偏向，是雌雄同体或雌雄异株的。这些结果在很大程度上得到了 PIC 分析的支持。不同的性系统在性别分配上提供了不同的灵活性，其中雌雄同体的灵活性最小，雌雄异株的灵活性最大。在 Ifloga 中，异常的头状花序是在一年生、雌雄同株的祖先中进化而来的，随后是两次雌雄异株的独立获得。在一个雌雄同株的亚系中，随后又发生了两次多年生性的获得。
不同的生活史具有不同的性别分配最佳值，并且与头状花序雏菊的不同性系统密切相关。Ifloga 中异常的头状花序结构可能促进了雌雄异株的进化，而雌雄异株的进化又可能与该属生活史多样性的进化有关。
