摘要：
自然杂交在植物界中十分普遍,一直是物种形成和进化研究的热点.秋海棠属(Begonia)为世界最大植物属之一,其种类繁多,但关于自然杂交的相关报道很少.系统调查秋海棠属的自然杂交不仅有利于理解该属植物的多样性,也是弄清其资源和解决物种分类问题不可缺少的环节.本文通过对国产秋海棠属植物自然杂交现象的系统调查和分析,发现总计29种(占国产已知200种秋海棠的约l5％)参与了杂交,产生了31个自然“杂种”约50个居群.其中,掌叶秋海棠(B.hemsleyana)和粗喙秋海棠(B.longifolia)参与杂交程度最高,分别与另外的8种和7种秋海棠发生了杂交;而裂叶秋海棠(B.palmata)发生杂交的居群最多,达16处.自然杂交以云南(尤其是南部)发生频率最高,一共有20种参与,产生了31处杂交居群;台湾和广西次之.杂交多为单向发生,个体以F1代个体为主,尚未脱离亲本独立成种.野外传粉生物学初步观察发现:秋海棠的主要访花昆虫为食蚜蝇类,其次是蜜蜂科,但各自传粉特点及效率尚待进一步研究.杂种及疑似亲本的鉴定可通过形态学、分子生物学和人工重复杂交等综合方法实现.杂种的基因组大小(C值)等于或接近两亲本的平均值,误差通常小于10％,有助于辅助鉴定杂种及亲本.我们总结后认为杂交发生及杂种形成必须满足5个条件:(1)重叠或邻近分布;(2)花期相遇;(3)有效传粉媒介;(4)杂交亲和;(5)适宜种子萌发幼苗生长的小生境及气候条件.据此,我们预测了国产秋海棠属未来可能发生的约40个新杂交组合的可能亲本及地点.到目前为止,尽管所有已知的秋海棠杂种尚未形成真正的新物种,但是,为了满足科学研究及实践交流的需要,建议按照新分类群发表的严格要求,将“杂种”逐一开展调查研究并整理发表,并将其作为物种进化过程中的一个特殊分类单元对待.鉴于秋海棠属的杂种个体稀少且并未对亲本构成威胁,反而在一定程度上增加了类群的多样性,并为优良观赏品种的直接选育创造了机会,因此,国产秋海棠属种类丰富及杂种区应为就地保护的优先考虑范围之一.同时,鉴于秋海棠属杂交亲和性高,迁地保护栽培时应增大花期重叠种类的种植间距,避免发生自然杂交.%Natural hybridization is a very common phenomenon in plants and has continuously been a hotspot in the scientific research of speciation and evolution.As one of the largest genera in plants,Begonia has a large number of species.However,very few cases have been reported on its natural hybrids.A systematic investigation of natural hybridization of Begonia can not only improve understanding of plant diversity and germplasm resources of this genus,but also be a step towards solving its taxonomical issues.Through a comprehensive field survey and experimental analysis of natural hybridization in Chinese begonias,we found nearly 50 populations of 31 natural hybrids occurred among 29 species,accounting for 15％ of currently about 200 accepted species.Begonia hemsleyana and B.longifolia had the highest cross frequency and crossed with 8 and 7 other species,respectively.B.palmata contributed to 16 hybrid populations,the largest number in all individual species.The largest number of hybrids (31 hybrid populations from 20 parental species) occurred in Yunnan (mainly in the southern areas),followed by Taiwan and Guangxi.Hybridization between species was unidirectional in most cases,and the majority of hybrids were F1 individuals,which still rely on parents or hybridization zones and have not been established as a true species.Field observations showed that aphid flies,followed by members of the bee family (Apidae),were the most frequent visitors of begonia flowers,however,further studies are necessary to examine their behavior and the efficiency of pollination.The natural hybrids and their putative parents were identified using integrative methods of morphology,molecular research,and artificial crossing experiments.Genome size (C value) of hybrids was usually equal or close to the average of that of the two parents,with an error less than 10％,which could assist identification of hybrids and parents.In addition,we summarized the five conditions necessary for the occurrence of natural hybridization:overlapping or adjacent distributions,concurrent flowering (at least partially overlapping),efficient pollination media,cross compatibility,and suitable microhabitat and climate conditions for seed germination and seedling growth.Based on these,we also predicted future putative parents of about 40 new natural hybrids and possible hybridization sites.Thus far,although all the known natural begonia hybrids have not established as true species,it may be more appropriate to treat them as a special taxon of speciation and evolution,which should be thoroughly investigated and published referring to protocols for publishing a new taxon in the scientific research.Because the individuals of natural hybrids in Begonia are always sparsely distributed in small numbers,and hybrids are not harmful to its parents it increases plant diversity and the chances of obtaining a new ornamental cultivar by natural selection.Therefore,the priority of in situ conservation should be given to regions with natural hybrids and species-rich environments.Meanwhile,for the purpose of ex situ conservation,requiring sufficient planting distance between species should be considered in those species with overlapping flowering time to avoid possible natural hybridization due to high cross-compatibility,particularly in native begonias.
