Ontogenetic Differences in Heterostylous Plants and Implications for Development From a Herkogamous Ancestor

Abstract

Alternative ontogenetic pathways among heterostylous species of Rubiaceae may reflect differences in their evolutionary histories. In this study, measurements were taken at different developmental stages on a series of longstyled (LS) and short-styled (SS) buds of the heterostylous taxa Psychotria chiapensis, P. poeppigiana, and Bouvardia ternifolia (all Rubiaceae). Results indicated that modifications in growth rates of stamens relative to corollas in all three species led to differences in anther heights between LS and SS flowers. Distinct style heights for LS and SS flowers of P. chiapensis and P. poeppigiana originate in the earlier stages of bud development and are maintained as styles elongate at equal rates. This contrasts with B. ternifolia, which has differences in style heights resulting from unequal relative growth rates between floral morphs. The ''approach herkogamous'' floral morphology, defined by having stigmas positioned above anthers, has been proposed as a potential evolutionary precursor for heterostylous taxa. To examine this hypothesis, floral development of two species with approach herkogamous morphologies, Psychotria pittieri and P. brachiata, was compared to that of the three heterostylous taxa. Differences in the relative rates of style elongation for flowers of approach herkogamous versus heterostylous species predict additional steps in the original model for the evolution of heterostyly from an approach herkogamous ancestor. The diversity of heterostylous ontogenies found within Rubiaceae provides insight into potential evolutionary pathways for this sexual system in other angiosperm families.

Population‐level variation in the expression of heterostyly in three species of Rubiaceae: does reciprocal placement of anthers and stigmas characterize heterostyly?

“…In all populations, plants with LS and SS flowers were present in approximately equal proportions (A. Faivre, unpublished data). Similarly, all plants that we studied had typical heterostylous incompatibility systems (Faivre, 1998), even the few plants of B. ternifolia (SCM) and one of P. chiapensis that consistently produced nearly homostylous flowers (i.e., with little separation between stigma and anthers) (A. Faivre, unpublished data). It seems unlikely that these aspects of heterostyly would be firmly established in populations in transition to or from this sexual system.…”

“…Intramorph variation-In all heterostylous populations, plants of the same floral morph showed a range of anther and stigma heights , with significant differences among some plants (for details see appendices in Faivre, 1998). There were significant differences among plants of the same floral morph for essentially all other floral dimensions measured (exceptions are petal length [both populations] and stigma lobe length [SNP] of P. poeppigiana which did not vary among individuals of the same morph) (results not shown).…”

“…Among Rubiaceae studied, only in Psychotria suerrensis (Stone, 1995) are LS flowers larger than SS flowers. The relationships between floral morph and corolla size suggest that heterostyly may be under phylogenetic or ontogenetic constraints in Rubiaceae (but see Faivre, 2000).…”

Producción de néctar y flores como rasgos compensatorios al robo de néctar en Bouvardia ternifolia (Rubiaceae): experimentos de campo

“…Schltdl. (Rubiaceae) es un arbusto diestílico autoincompatible (a partir de ahora mencionada sólo como Bouvardia), mide de 1-1.5 metros de altura; y está ampliamente distribuido en áreas abiertas de los bosques de Arizona, EUA hasta el sur de México (Faivre, 2000). Las fl ores presentan las corolas fusionadas, en forma de tubo y agrupadas en cimas terminales.…”

Reproductive biology of the herkogamous vine Chiococca alba (L.) Hitchc. (Rubiaceae) in the Atlantic Rain Forest, SE Brazil

“…In distyly this separation is observed early in the bud stage (Faivre 2000), whereas in styllar pollen presentation it is a result of style elongation after anther dehiscence (Imbert & Corresponding author: cibelecastro@hotmail.com , Ladd 1994. Herkogamous species may be homomorphic or heteromorphic, depending on the number of floral types that are produced.…”