在自然界里，多数植物为雌雄同株，植物具有纷繁复杂的机制以避免自交或促进异交，雌雄异位就是其中一种机制。柱高二态是被子植物中特殊的花内雌雄异位的一种类型，指的一个居群类存在两种花型植株，它们的花柱高度显著不同但花药高度却接近。柱高二态现象并不普遍，其中茜草科是被子植物中异型花柱最为集中的科，所包含的具异型花柱属、种数量超过其它所有科之和。
中科院华南植物园植物系统与进化生物学研究领域李爱民博士后与吴小琴博士，在对茜草科玉叶金花属的玉叶金花（Mussaenda pubescens Ait. f.）繁育系统进行研究时发现，该植物表型上符合柱高二态的特征，但野外观察发现其长柱植株开花时花药萎缩，短柱植物不结实。进一步对玉叶金花大小孢子及雌雄配子体发育观察发现，其长柱花大孢子及雌配子体发育正常，最终形成可育胚囊；由于花药绒毡层提前解体，导致长柱花小孢子及雄配子体发育不正常，不能形成可育的花粉，即雄性不育；而短柱花的大孢子及雌配子体发育的各个阶段均出现退化现象，不能形成功能性大孢子或正常胚囊，即雌性不育。因此，玉叶金花是表型上的柱高二态，实际繁育系统为功能性雌雄异株。
这个实例并不符合以往认为的柱高二态是异型花柱进化过程中不稳定的中间阶段这一论断。对此，作者首次提出了异型花柱并不是柱高二态进化的唯一途径，柱高二态可以直接进化为雌雄异株的新观点。该项研究结果已发表在近期出版的国际著名学术期刊Annals of Botany上。
推荐原文出处：
Annals of Botany, doi:10.1093/aob/mcq146
Cryptic dioecy in Mussaenda pubescens (Rubiaceae): a species with stigma-height dimorphism
Ai-Min Li1,2,, Xiao-Qin Wu1,, Dian-Xiang Zhang1,* and Spencer C. H. Barrett3
1 Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, The Chinese Academy of Sciences, Guangzhou 510650, China
2 Department of Life Sciences, Huaihua College, Huaihua 418008, China
3 Department of Ecology " Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada M5S 3B2
Background and Aims: Evolutionary transitions from heterostyly to dioecy have been proposed in several angiosperm families, particularly in Rubiaceae. These transitions involve the spread of male and female sterility mutations resulting in modifications to the gender of ancestral hermaphrodites. Despite sustained interest in the gender strategies of plants, the structural and developmental bases for transitions in sexual systems are poorly understood.
Methods: Here, floral morphology, patterns of fertility, pollen-tube growth and floral development are investigated in two populations of the scandent shrub Mussaenda pubescens (Rubiaceae), native to southern China, by means of experimental and open-pollinations, light microscopy, fluorescence microscopy and scanning electron microscopy combined with paraffin sectioning.
Key Results: Mussaenda pubescens has perfect (hermaphroditic) flowers and populations with two style-length morphs but only weak differentiation in anther position (stigma-height dimorphism). Experimental pollinations demonstrated that despite morphological hermaphroditism, the species is functionally dioecious. The long-styled (L) morph possesses sterile pollen and functions as a female, whereas the short-styled (S) morph is female sterile and functions as a male. Self- and intra-morph pollinations of the S-morph were consistent with those expected from dimorphic incompatibility. The two populations investigated were both S-morph (male) biased. Investigations of early stages of floral development indicated patterns typical of hermaphroditic flowers, with no significant differences in organ growth between the floral morphs. Meiosis of microspore mother cells was of the simultaneous type with tetrads isobilateral in shape. The tapetal cells in anther walls of the L-morph became vacuolized during meiosis I, ahead of the uninucleate microspore stage in the S-morph. In the L-morph, the microspore nucleus degenerated at the tetrad stage resulting in male sterility. Microsporogenesis and male gametophyte development was normal in the S-morph. Failure in the formation of megaspore mother cells and/or the development of megagametophytes resulted in female sterility in the S-morph, compared with normal megasporogenesis in the L-morph.
Conclusions: In M. pubescens, cryptic dioecy has evolved from stigma-height dimorphism as a result of morph-specific sterility mutations.
