Reciprocal style polymorphisms are not easily categorised: the case of heterostyly inLithodoraandGlandora(Boraginaceae)

Abstract

Interest in reciprocal floral polymorphisms, such as heterostyly, has increased in recent decades because they can be used as suitable model systems to study mechanisms of outbreeding and disassortative mating in plants. Heterostylous plants are characterised by the presence of discrete morphs that differ in sex organ position and in some other ancillary traits. As regards sex organ deployment, different types of polymorphisms have been described, depending on number and type of discrete classes present in populations and degree of reciprocity between them. However, a clear-cut characterisation of stylar polymorphisms does not appear to be the best approach when there is great variability among populations because of continuous variation of some of traits examined. A recent study in Lithodora sensu lato (recently split into two separate genera, Lithodora and Glandora) showed a wide variation in sex organ position across species in the genus, which warrants precise population analysis of stylar polymorphism and its reciprocity. We provide a detailed morphometric analysis of flower sexual traits and include those considered to be ancillary characters. We report a wide variation in these traits in populations of Lithodora s.l. and highlight the subjectivity of the former characterisation of style polymorphism based on visual inspection. Ancillary traits appear repeatedly in Lithodora and Glandora, particularly in the latter. The appearance of these traits seems to be related to greater reciprocity between sexual whorls in Glandora, with the exception of G. prostrata. These results agree with evolutionary steps proposed in the build-up of heterostyly according to some evolutionary models. We also examined variation in polymorphisms in light of current models for evolution of heterostyly, and, more specifically, we sought to verify the prediction that flower traits as a whole (i.e., flower integration) respond to selective pressure to assure the exact location of pollen on the pollinator body. Most reciprocal populations and species, where between-morph pollen transfer is expected to be higher, would show greater integration. Our results confirm this hypothesis.

Associations between sex‐organ deployment and morph bias in related heterostylous taxa with different stylar polymorphisms

“…In these species sex-organ reciprocity is the primary mechanism promoting disassortative mating and indeed there is evidence that this may occur. Glandora moroccana and G. diff usa are fully self-and intramorph compatible with high sex-organ reciprocity ( Ferrero et al, 2011asee Fig. 5 ) and all populations sampled were isoplethic ( Fig.…”

“…We chose this group for several reasons. First, our earlier investigations revealed considerable variation in both sex-organ reciprocity and compatibility systems, including species with classical distyly with heteromorphic incompatibility, distyly with self and intramorph compatibility and, fi nally, stigma-height dimorphism with self-incompatibility and intramorph compatibility ( Ferrero et al, 2009( Ferrero et al, , 2011a. To our knowledge, this range of reproductive variation surpasses that found in any dimorphic group of closely related species.…”

“…), following procedures detailed in Ferrero et al (2009Ferrero et al ( , 2011a. We then applied the method of Sánchez et al (2008Sánchez et al ( , 2013 to calculate an index of reciprocity ( R ) between complementary sex organs in each population.…”

“…For this, we characterized the taxa into two groups based on their degree of reciprocity: distylous populations, which generally display greater levels of reciprocity, and nondistylous populations, including those with stigma-height dimorphism and relaxed stigmaheight dimorphism, which exhibit weaker reciprocity ( Ferrero et al, 2011a ). Th en, we calculated a mean morph bias per taxon.…”

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).…”