Viola cazorlensis

 

 

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Viola cazorlensis Gand. (Violaceae) is a perennial, suffruticose violet endemic to a few limestone mountain ranges in south-eastern Spain (see Proyecto Anthos). This species, along with two closely related taxa occurring in the Balkan Peninsula (V. delphinantha and V. kosaninii), constitute the small section Delphiniopsis of the genus Viola, an old and apparently primitive lineage. It occurs as small, discrete populations, almost invariably associated with scattered rocky outcrops, cliffs, and pockets of sandy soils originating from heavily weathered limestone on mountain ridges. Populations are largely variable in size.

Our early studies focused on floral morphology and the interaction with pollinators. The pinkish-purple flowers are characterized by a long spur (up to 37 mm in length, mean = 25.1 mm), where nectar accumulates at the distal portion (0.24 0.28 ml per flower). They vary widely in morphology within and among populations. Details on floral morphology variation can be found here. Flowers are self-compatible, although only about 1% of excluded flowers are able to produce fruits autonomously, and the action of pollinators increases dramatically fruit-set. The main pollinator is a day-flying hawk moth, Macroglossum stellatarum (Lepidoptera, Sphingidae). The morphology of a flower does not determine the probability of setting fruit, although peduncule length largely influenced the number of seeds/fruit. Plant seed production did however depend more of total flower production. Thus, floral variability in this species may be explained by the combined action of selection on peduncle length (the character most variable among individuals) and little stabilizing selection on spur length and size of petals (see here). This was one of the first study cases in which directional selection of pollinators on corolla or spur length was not verified (see also here). Studies using a population genomic approach have shown that among-population variation in floral characteristics observed in this species reflects adaptive divergence driven by geographically variable selection (read more). In addition, adaptive genetic divergence of populations is significantly correlated with epigenetic differentiation, as shown by a study on patterns of DNA methylation (read more).

Although individual differences in fecundity are influenced by extrinsic factors, such as microhabitat type and interactions with herbivores, a sizeable fraction of individual differences in long-term maternal fecundity can be explained by genetic differences alone (read more). Type of substrate (rock cliffs, bare rocks at ground level, and sandy soils) largely determines differences between V. cazorlensis plants in the impact of herbivory. Plants growing on the soil are those incurring the greatest reproductive losses to herbivores. Also cliff plants tend to be the largest and rock plants the smallest. Plant size, in turn, influences the supra-annual frequency of flowering and the number of flowers produced in each reproductive event. Furthermore, the proportion of reproductive individuals within a population decreases in years with low precipitation in January-March (the three months previous to floral differentiation). As a consequence the influence of floral morphology in individual differences in fecundity is relatively small (see here for more details).

In a long-term study (1988-2003) in a single population, both the proportion of flowering individuals per year and the average number of fruits per plant showed a negative temporal trend, indicating a progressive reduction of the population reproductive success that was particularly strong in plants growing in the soil. Among the potential causes of such pattern herbivory and winter drought appeared as the strongest determinants of reproductive decline. Consumption by ungulates showed the opposite temporal trend and both the proportion of individuals affected and the individual percentage of biomass consumed tended to increase in the second half of the study period, causing a reduction in subsequent flower production. Again plants growing at ground level were more negatively affected by herbivores. Winter (January – March) precipitation was relatively low within the 16 yr study period as a consequence of a progressive reduction of precipitation in those months observed in the period 1955-2003. Reduced rainfall did also negatively affect fruit production within the study period, thus contributing to the observed reduction in plant reproduction success that could impose an increasing risk to the conservation of this endemic violet.


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