科研之友
微信
新浪微博
Facebook
分享链接
摘要
Cuscuta (dodder) is one of the few angiosperm genera characterised by multiple fruit indehiscence/dehiscence modes. Dodder fruit is a capsule with thin pericarp and contains one to four seeds. In some species fruits open at the base (circumscissile dehiscence; DE) while in others they remain closed (indehiscent; IN). IN has evolved multiple times from DE, and is most common in the North American clades of subgenus Grammica. In addition, different forms of "irregular dehiscence" have been anecdotally reported. To determine fruit types and assess homology, we explored fruit development, structure and ultrastructure together with a comparative morphology and character evolution study of other fruit traits. Because it is commonly assumed that IN is associated with a loss of dispersal capability when diaspores contain several seeds, we studied the geographical distribution of species with DE and IN fruits in North America to ascertain whether the latter are less widely distributed than DE species. To determine the putative dispersal advantage(s) of IN we studied the floatability of both capsules and seeds of C. gronovii, a species with IN fruits. Also, in this species we examined the effects of the fruit on seed germination. Pericarp structure/ultrastructure revealed three distinct fruit types: DE, with an abscission zone (AZ); IN without an AZ, and irregularly dehiscent capsules (IrA) which lack an AZ but open because of the thin endocarp cell walls at the fruit base. A fourth functional fruit type, the irregularly dehiscent type B capsules (IrB) was noted, in which dehiscence apparently occurs because of mechanical pressures created among ripening IN fruits within the dense infructescences of the same plant. IrA fruits were posited to be an intermediate evolutionary step between DE and IN fruits; IrA and IrB fruits are not homologous. The only statistically significant fruit trait associated with the dehiscence mode is the size of the interstylar aperture which is larger in IN fruits than in DE capsules. Although not statistically significant, species with IN fruits appear to be more widely distributed in N America that species with DE capsules suggesting an enhanced dispersal capability of IN fruits. Also, species with IN fruits possess a significantly higher northern latitudinal limit in North America. Capsules of C. gronovii were capable of floating for at least one week longer than their seeds indicating that long-distance dispersal by water is possible under certain conditions. Seeds of C. gronovii not removed from their IN fruits exhibited a delayed germination pattern compared to removed seeds, which heightened germination be hedging and may prevent sibling competition for the same host plants.
- 出版日期2018-6
