Variation in pollen sterility and gender specialization: an investigation with distylous species of Erythroxylum (Erythroxylaceae).

Differences in pollen sterility between morphs in distylous populations may represent the first step in the evolution of gender specialization. Theoretically, for partially or completely male sterile individuals to persist they must have some fitness advantage that compensates for the loss of male function. Gender specialization is considered a widespread process in Erythroxylum, but male sterility and the resource reallocation to female function have been investigated in few species and populations. In 18 populations of four distylous species of Erythroxylum, we quantified the levels of male sterility, estimated through pollen sterility, in short- and long-styled flowers to test if sterility is morph-biased. In one population per species, we also described the frequency of floral visitors, the production of flowers and fruits, and the quality of fruits and seeds of short- and long-styled plants to evaluate the expression of trade-offs in allocation to male and female function. In some populations of E. campestre and E. deciduum, short-styled flowers possessed higher levels of pollen sterility than long-styled flowers. Although most flowers of E. suberosum and E. tortuosum also expressed pollen sterility, the frequency of sterility was similar between morphs in all populations. Differences in reproductive output between morphs occurred only in populations of species with morph-biased sterility, but none of the variations reflect resource allocation to female fitness of short-styled plants. Differences in the level of sterility between morphs indicates the potential for gender specialization in populations of E. campestre and E. deciduum, despite the apparent lack of a trade-off in allocation.

Style polymorphism in Linum (Linaceae): a case of Mediterranean parallel evolution?

Heterostyly is a sex polymorphism that has challenged evolutionary biologists ever since Darwin. One of the lineages where heterostyly and related stylar conditions appear more frequently is Linum (Linaceae). This group is particularly suitable for testing competing hypotheses about ancestral and transitional stages on the evolutionary building up of heterostyly. We generated a phylogeny of Linum based on extensive sampling and plastid and nuclear DNA sequences, and used it to trace the evolution of character states of style polymorphism. We also revised available data on pollination, breeding systems, and polyploidy to analyse their associations. Our results supported former phylogenetic hypotheses: the paraphyly of Linum and the non-monophyly of current taxonomic sections. Heterostyly was common in the genus, but appeared concentrated in the Mediterranean Basin and the South African Cape. Ancestral character state reconstruction failed to determine a unique state as the most probable condition for style polymorphism in the genus. In contrast, approach herkogamy was resolved as ancestral state in some clades, giving support to recent hypotheses. Some traits putatively related to heterostyly, such as life history and polyploidy, did show marginal or non-significant phylogenetic correlation, respectively. Although pollinator data are limited, we suggest that beeflies are associated with specific cases of heterostyly. The consistent association between style polymorphism and heteromorphic incompatibility points to ecological factors as drivers of the multiple evolution of style polymorphism in Linum. Albeit based on limited evidence, we hypothesised that specialised pollinators and lack of mating opportunities drive evolution of style polymorphism and loss of the polymorphism, respectively.

The function of the floral corona in the pollination of a Mediterranean style dimorphic daffodil.

Narcissus papyraceus is a style dimorphic species with two floral forms, with anthers at similar height and stigmas above (long-styled L) and below (short-styled S) the anther level. The species is self-incompatible, but intra- and inter-morph compatible. Populations are either dimorphic (including both morphs) in the region of the Strait of Gibraltar, or L-monomorphic (with only L plants) in the inland of the Iberian Peninsula. This variation correlates with the most common floral visitors, being primarily long-tongued and short-tongued pollinators, respectively, a rare condition in Mediterranean plants. The maintenance of S-flowers relies on long-tongued insects, as only those deliver pollen to short-styled stigmas. Narcissus flowers present a long and narrow tube, at the bottom of which nectar accumulates, and a floral corona, which has been proposed as an important trait for the attraction of pollinators. Here we tested the importance of the corona on pollination of L and S flowers. We described UV reflectance patterns of the corona and tepals, and characterised VOCs in intact flowers and flowers with trimmed coronas. We also conducted a field experiment in the dimorphic and monomorphic region to estimate the importance of corona removal on seed production in stands with solitary plants and in groups to control for compatible pollen limitation. Reflectance was higher in the tepals than the corona, although both traits presented a reflectance peak around 450 nm wavelength. L- and S-flowers produced similar volatiles, regardless of the manipulation of the corona. Across dimorphic and monomorphic regions, S-flowers with the corona removed suffered a reduction in seed production of ca. 50%, while seed production remained similar in L flowers both with the corona intact and removed. Plants in solitary stands suffered a strong reduction in seed production, which was more pronounced in the monomorphic region. Our results suggest that the corona in Narcissus is more important for the pollination of S-flowers, which generally have lower seed production compared to L-flowers. Taken together, these results suggest that the floral corona indirectly plays an important role for maintenance of the polymorphism.