Floral Development at Multiple Spatial Scales in Polygonum jucundum (Polygonaceae), a Distylous Species with Broadly Open Flowers

Abstract

Distyly, a special polymorph, has evolved in many groups of angiosperms and has attracted attention since Darwin’s time. Development studies on distylous taxa have helped us to understand the evolutionary process of this polymorph, but most of these studies focus on species with narrowly tubular corolla. Here, we studied the floral development of Polygonum jucundum, a distylous species with broadly open flowers, at multiple spatial scales. Results showed that the difference in stigma height between flowers of the two morphs was caused by differences in style growth throughout the entire floral development process. The observed difference in anther heights between the two morphs was because the filaments grew faster in short-styled (SS) than in long-styled (LS) flowers in the later stages of floral development. In addition, the longer styles in LS flowers than in SS flowers was because of faster cell division in the early stages of floral development. However, SS flowers had longer filaments than LS flowers primarily because of greater cell elongation. These results indicate that floral development in P. jucundum differs from that of distylous taxa with floral tubes shown in previous studies. Further, we conclude that the presence of distyly in species with open flowers is a result of convergent evolution.

How to build distylous flowers: comparative floral development and evolution of distylous species across the angiosperms

“…), so it is unusual to observe this increase in later‐stage growth. This pattern for anther elevation has only been noted previously in distylous species of Polygonaceae, in which the later growth rate of the filaments, particularly the outer whorl of filaments (Huang et al., ), increases in the SS morph compared to the LS morph (Tables ), and in the tristylous species Eichhornia paniculata Solms (Pontederiaceae) (Richards and Barrett, ). In the other species, N. stenosiphon , the organs contributing to anther height in the SS morph appear have a longer period of growth than in the LS morph, which also occurs in some species of Houstonia .…”

“…Two are included in the present study, Polygonum jucundum Meisn. (Huang et al., ) and Fagopyrum esculentum , and both arose from an ancestor lacking herkogamy, providing evidence of similar pathways for the evolution of the breeding system across the family. However, in P .…”

“…A, B). Corolla length was measured for all flowers, and corolla length serves as a proxy for flower age (i.e., time) (Cohen et al., ; Huang et al., ). All measurements were taken from the base of the corolla, point of filament attachment, or base of the organ measured (Fig.…”

“…Patterns of floral development in the two morphs differ among the independent origins of distyly (Richards and Barrett, ; Cohen, ). The morph‐specific anther and stigma heights at anthesis can result from various developmental mechanisms, with differences between morphs observed at the genomic (Nowak et al., ), transcriptomic (McCubbin et al., ; Cohen, ), micromorphological (Cohen et al., ; Huang et al., ), and morphological levels (Richards and Barrett, ; Richards and Koptur, ; Faivre, ; Armbruster et al., ; Cohen et al., ; Huang et al., ), and examinations at all levels aid in the understanding of differences among independent origins of the breeding system. Investigations at the most macroscopic of these levels—morphological—provide insight into the multiple manners in which the same organs can attain different lengths and heights in the flowers of each morph, including different (1) growth rates throughout development; (2) growth rates during particular times of development; (3) organ, structure, and meristem sizes during early development; (4) times of organ initiation and cessation; (5) duration of organ growth or combinations of these possible patterns (Fig.…”

“…Patterns of floral development between the morphs of distylous species differ across the angiosperms (Richards and Barrett, ; Richards and Koptur, ; Faivre, ), and even among members of a medium‐sized genus, Lithospermum (Boraginaceae), divergent patterns of morph‐specific floral development have arisen (Cohen, Litt, and Davis, ). To date, however, floral development has only been examined in a limited number of distylous species (Riveros et al., ; Richards and Barrett, ; Richards and Koptur, ; Faivre, ; Webster and Gilmartin, ; Li and Johnston, ; Cohen et al., ; Sampson and Krebs, ; Huang et al., ; Bull‐Hereñu et al., ), and only rarely have multiple species been included in a single study. The present study is an examination of comparative floral development of the two distylous morphs for 15 species from across the angiosperms.…”