Comparative floral development in Lithospermum (Boraginaceae) and implications for the evolution and development of heterostyly

Abstract

Floral developmental patterns in homostylous species provide evidence that modification of specific traits, such as patterns of stylar growth, can lead to the evolution of heterostyly. The developmental changes that affect the positions of the stigmas and anthers in each morph likely involve either temporal or spatial modifications of gene function. The floral developmental patterns described here and the occurrence of multiple types of herkogamy within some species of Lithospermum provide evidence that heterostylous species in the genus have originated via distinct evolutionary developmental pathways.

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

“…Thus, the filament made little contribution to the anther height in these species [10], [16], [31]. However, in P. jucundum , the stamens are independent from the separate tepals and they possess long filaments.…”

“…Cell division and cell elongation could contribute to the growth of floral organs [11], [16], [20]. Different microscopic patterns of floral development were found in some distylous species with tubular corolla, which showed cell division mainly served to influence anther height in two morphs.…”

“…It is thought that either the dimorphic height of floral organs evolves after the appearance of self-incompatibility in homostylous ancestor [13] or distylous taxa evolve from a self-compatible ancestor with a herkogamous floral morphology [6]. The evolutionary process of some distylous taxa has been revealed by investigations into the development of floral organs [10], [14]–[16]. In the floral development of these distylous species, four different development pathways are known to cause differences in stigma heights between LS and SS flowers.…”

“…Only two different development pathways are observed in the divergency of anther heights between two types of flowers. The relative growth rate of buds is very important in these two pathways [10], [16]–[19]. These developmental differences suggest that a stigma-height mutation is the probable cause of distyly and that the ancestors of these distylous taxa have “approach herkogamy” or “reverse herkogamy” with stigmas borne at a different level to the anthers [10], [16].…”

Distylous traits in Cordia dodecandra and Cordia sebestena (Boraginaceae) from the Yucatan Peninsula

“…also differ in ancillary traits, such as pollen grain size and number, stigmatic papillae morphology, stigma shape and corolla size (Dulberger, 1992;Richards, 1997;McCubbin, 2008;Ferrero et al, 2011;Cohen et al, 2012). Of the fl oral traits involved in distyly, reciprocal herkogamy is the main morphological component shaping fl oral morphs (Ganders, 1979).…”