Sex Expression and the Evolutionary Advantages of Male Flowers in an Andromonoecious Species, Sagittaria guyanensis subsp. lappula (Alismataceae)

冠果草的性表达状态及其进化含义(英文)

Sex expression in Sagittaria guyanensis H.B.K. subsp. lappula (D. Don) Bojin is typically andromonoecious while the other species in the same genus are basically monoecious. The evolutionary advantages of male flowers and hermaphrodite flowers in S. guyanensis subsp. lappula were assessed by measuring sex allocation and pollen movements in two wild populations of the species. Two cultivated populations served as controls. The percentage of male flowers was very low in the two wild populations in Dongxiang, Jiangxi Province and Wuyishan, Fujian Province, viz., 2.48% and 0.96% respectively. In the two cultivated populations, male flower percentage significantly increased when the soil was of higher nutrient content. This indicates that the allocation to male versus female reproduction might change in response to environmental factors. Pollen production per male flower was 4.1 times higher than that of a hermaphrodite flower. The floral shape and size of male and hermaphrodite flowers were similar. No difference was observed between these dimorphological flowers in pollen germination rate in vitro and in the speed of pollen tube growth in vivo . Anthesis was only 4-5 h. Male flowers usually opened 0.5 h earlier than hermaphrodite flowers. An unexpected finding was that no pollen from the male flowers was found on the stigmas of the hermaphrodite flowers, in spite of the occasional visits by insects to both types of flowers in both wild and control populations. A consistent pattern of fruit development was found to exist in open pollinated flowers as well as in flowers that had been bagged. The sex ratios did not have significant influence on fruit set. Approximately 25% of the pistils in a gynoecium failed to develop into fruits because no pollen was deposited on them, indicating that the fruit set of this andromonoecious plant is mainly affected by pollen limitation rather than resource limitation. Reproduction in S. guyanensis subsp. lappula in the habitats was dependent on self_pollination in hermaphrodite flowers. The male flowers in this species might be a potential source of additional pollination and may facilitate cross_pollination. The fact that the flowers of monoecious species in Sagittaria pollinated by a wide diversity of insect visitors may contribute enormously to the diversification of sex expression in this genus.

通常认为雄花两性花同株是植物性表达从两性花进化到雌雄异株的过渡类型之一。慈姑属仅冠果草(SagittariaguyanensisH .B .K .subsp .lappula (D .Don)Bojin)的性表达为典型的雄花两性花同株 ,其他种为雌雄同株。对冠果草野外和实验居群的性配置进行了定量观察。东乡和武夷山居群雄花的比例分别为 2 .48%和 0 .96 %。实验表明 ,栽培条件影响冠果草的性表达 :在营养较好的条件下 ,产生花序大且多 ,雄花的比例显著提高 (2 6 .78% ) ;在营养不利时则相反 ,冠果草个体小 ,产生花序少 ,雄花数量少 (4 .91% )。雄花的花粉产量是两性花的 4.1倍。两者的花粉活力和在柱头上的萌发能力没有明显差别。单花期为 4～ 5h ,同一天的开花时间上 ,雄花比两性花早约0 .5h。尽管单个个体雄花比两性花少 ,但在居群水平上雄花与两性花的比例呈现大幅度波动。但是雄花并未作为花粉供体存在 ,野外和实验居群都极少有昆虫访花。大量统计表明 ,不同居群仅 2 .1%～ 3.2 %的果实发育不全 ,性比对结实率没有显著影响。 1/ 4左右的柱头因未授粉而不发育 ,影响冠果草结实的主要因素是花粉限制而不是资源限制。两性花套袋的结实率与自然传粉相当。冠果草在目前生境基本是通过两性花进行自花授粉 。