|其他摘要||The genus Salix (Salicaceae) is widely distributed in the world excetp Australia and Antarctic, mainly in temperate zone of North Hemisphere, with about 520 species all over the world and 275 in China.
In this paper, we have primary studied and discussed the systematics and biogeography of Salix based on the research findings in the fields of morphology, leaf epidermis, cladistic analysis, molecular systematics. Meanwhile, we have presented some primary results in evolutionary and biogeographical relationships between Salix sections Lindleyanae and Retusae. The most important findings are summarized as follows:
1. Leaf epidermis
42 species of the family Salicaceae were sampled for studying their leaf epidermal characters under light microscope (LM) and scanning electro microscopy (SEM). The results indicated that the types of stomatal apparatus in Salix or Salicaceae were reltively diverse, including paracytic, laterocytic (3 subtypes), anomocytic, cyclocytic stomatal apparatus observed in Salicaceae. Anomocytic stomata was found only in sections Lindleyanae and Retusae, but it was still unclear whether the epidermal similarity of these two sections indicated their similar habitat conditions and/or their same origin in phylogeny. Cyclocytic sotmata was observed only in Populus, Chosenia sect. Urbanianae (i.e. Toisusu Kimura), which to some extent supported the segregation of Chosenia and Toisusu from Salix as two independent genera. While paracytic and laterocytic stomatal types were observed in almost all species of Salix. Paracytic stomata was considered to be primitive in Salix and Salicaceae and other stomata types were derived from this type.
The shape of leaf epidermal cells of Salix are mostly polygonal, seldom irregular; The anticlinal walls of epidermal cells are mostly straight to curved, seldom repand and sinuous; The inner margin of the outer stomatal rim was nearly smooth or erose to repand; The cuticular membrane of the leaf epidermis was striated, often with striated, scales, grains or conicoids wax filaments ornamentation; Stomatal size and density varied in response to enviromental factors. Although these characters are of little value to Salix systematics, they are stable enough for diagnostic use.
2. Cladistic analysis of morphological data
Sixty-one morphological characters were used for cladistic analysis, the result indicated that Salix is monophyletic and Chosenia and the division of Chosenia and Toisusu (sect. Urbanianae) from Salix as independent genus is unreasonable. But the resolution and support within Salix clade of the topology were relatively poor. Little clades gained over 50% bootstrap value. Sect. Retusae was monophyletic and then formed monophyletic together with sect. Lindleyana , which indicated those two sections probably had a close relationship.
The Constant Index of the strict consensus tree was only 0.214 and 0.245, which indicated too many non-homoplasy characters among the character selected for cladistic analysis, and low reliability and veracity of system reconstruction.
3. Molecular systematics
Molecular Phylogeny of 29 sections and 38 species of Salix was analyzed based on chloroplast trnD-T, atpB-rbcL noncoding spacer and rbcL gene sequence data separately and collectively. The result revealed that Salix is monophyletic with support of high bootstrap value, and indicated any subdivision of Salix in the generic level was not supportive and acceptable and two genera, Chosenia and Toisusu, should merge into the genus Salix.
The research findings indicated that Salix can be divided into 2 main clades, one included all species of subgenus Salix (system of Skvortsov, 1968), but the species from North America and Old World were separated remarkably wthin this clade. While in the other clade, sections Amygdalinae was in the basal position, Chosenia and Urbanianae had close affinity and in the sub-basal position. And all species of subgenus Chamaetia and Vetrix formed a large clade in the top position, and the inner resolution of these calde was very low, which indicated that these two subgenera had a close affinity and they might evolve by radiative speciation. The result also revealed that both sect. Lindleyanae and Retusae were paraphyletic.
Molecular systematics findings indicated the ancestor taxa of Salix had experienced divergency soon after they originated and evolved at least two clades. One of them was the ancestor of sect. Amygdalinae, Chosenia, Urbanianae, subgenus Chamaetia and Vetrix. While one another evolved subgen. Salix.
Based on moleculars systematic findings we complement the Salix system on subgeneric level, we suggest that Salix divided into five subgenus, subgen. Salix, subgen. Amygdalina, subgen. Chosenia, subgen. Chamaetia, subgen. Vetrix.
4. Primary discussions on biogeographical analysis
There are 3 distribution centers in the genus Salix, Esat Asia, Iran-Turanian, Circum-North. Esat Asia especially Himalaya-Hengduan mountain region are distribution center and divergency center of Salix. Moreover, Salix originated in north-east Asia, no later than late Early- Cretaceous.
After Salix origined, it was probably diverged into at least two primitive ancestor taxa. One dispersed towrds east and west and finally met in North America, and later those primitive taxa from North America had almost no gene exchange with those of Old World and both of them evolved independently. The Salix of South America was spread from North America in Pliocene when North America and South America was connected; Another primitive ancestor taxa of Salix was furthered evolved out taxa include subgenus Amygdalina, Chosenia, Chamaetia and Vetrix.
Ancestor taxa of subgenus Chamaetia moved to south of Eurasia when the global climate became cold in the Tertiary, and dispersal into highlands of South-West China and began their speciation as the uplift of Qinghai-Tibet Plateau started and finally evolved the mostly Salix taxa of Qinghai-Tibet Plateau include sect. Lindleyanae. Taxa which have same distribution pattern with sect. Retusae were of Arctic-Tertiary origin and late spread into middle latitude area of North Hemisphere and move back to high latitude when climate turned warm, after several times of this process the Arctic-Alpine distribution pattern of Retusae and also some other sections were formed. Consequently, sections Lindleyanae and Retusae have no direct affinity, section Lindleyanae have comparatively independent way of origin and dispersal.|