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Daphne laureola L. (Thymelaeaceae) is an evergreen shrub growing in the undergrowth of shady mountain forests. It has a Palaearctic distribution ranging from the Atlas Mountains, in Morocco, northwards to England and Hungary. In the Iberian Peninsula, the species is abundant in the central and western Cantabrian Range, the Pyrenean Mountains, and the southern Betic Ranges. It is infrequent in the eastern Cantabrian Range, and absent from the central Iberian Ranges (check for the species' distribution at www.anthos.es).
We became interested in the interactions with animals of this species for at least three different reasons: nocturnal and unspecialized herbivores, winter flowering and a particular polymorphic breeding system: gynodioecy (the coexistence of hermaphrodite and male-sterile individuals within populations).
Herbivory: Diurnal leaf consumption has been never recorded in our study area in Cazorla. There, polyphagous noctuid caterpillars (Lepidoptera: Noctuidae) are the only herbivorous insects observed feeding on leaves, flowers, green fruits and seedlings. The most abundant species are Noctua janthe, N. fimbriata, Trigonophora flammea and Pseudenargia ulicis, and their relative abundance vary from year to year. Our studies in Cazorla have demonstrated that individual plant size and architecture determined the abundance of noctuid caterpillars (see here), also the nutritious value of the leaves was positively related to leaf defoliation (see here). We are currently studying the role of some specific secondary compounds (the coumarins) in this interaction. Coumarins are a group of phenolic compounds, widespread throughout the plant kingdom that function as UV-absorbent screens, protecting plants from damaging radiation, may inhibit seed germination and growth, and are toxic to a wide variety of organisms including bacteria, fungi, invertebrates and vertebrates. In Cazorla, concentration of coumarins in fully-developed leaves of D. laureola increased with altitude (see details here) and this pattern is much more evident in female plants. If you want to know more about the behavioural responses of the caterpillars to these secondary compounds check here.
Pollination: Early blooming species in seasonal environments require extended flowering, unspecialized pollination, or both to succeed in reproduction. We have seen that extended flowering contributes significantly to both reproductive success and maintenance of gynodioecy in Daphne laureola. However, pollination is relatively specialized and the only effective pollinator is a beetle that prefers sunny locations, hence emphasizing the relevance of abiotic conditions for reproductive success of this early blooming shrub (see here). If you want to know more about the consequences in terms of pollination success of the solar irradiance environment experienced by individual plants check here.
Gynodioecy: This polymorphic reproductive system (hermaphrodite and female individuals coexist within populations) has been traditionally viewed as one intermediate evolutionary stage between the two most common reproductive systems of Angiosperms, hermaphroditism and dioecy. In order to be maintained within populations females must have some consistent fecundity advantage that compensates for their gametic disadvantage. Our initial studies on D. laureola in Sierra de Cazorla indicated absence of differences between morphs in many traits including size and seed production (see here). Further we demonstrated a very high rate of selfing in hermaphrodites in several populations differing in female frequency coupled with strong inbreeding depression, suggesting that females could be maintained due to their inbreeding avoidance advantages in Southeaster Iberian populations (see here). Consistently, we have found that totally hermaphroditic populations exist in SW and NE Iberian populations characterized by higher temperatures and lower precipitation than the gynodioecious ones (see here). Plants in the hermaphroditic populations of the NE region are characteristically small and also recorded higher pollination success, two aspects that could reduce geitonogamy and the advantages of inbreeding avoidance, helping to maintain hermaphroditism. The analysis of genetic relationships between populations geographically distant and with different breeding system supports D. laureola as a monophyletic lineage with three different clades within the Iberian Peninsula, as shown here. The NE and S hermaphroditic populations belong to two different clades, whereas gynodioecy is ubiquitous but characteristic of the third clade, which grouped together all the NW Iberian populations studied, and include the apparently oldest haplotype sampled. Gynodioecy appears as the most likely basal condition of the 13 analyzed populations, but different evolutionary transitions in reproductive sexual system were traced within each D. laureola clade, indicating that the evolution of gynodioecy in plants can be a two-way street (further reading).
Geographic variation of plant-animal interactions: glancing at range distribution limits: Distribution margins constitute areas particularly prone to random and⁄or adaptive intraspecific differentiation in plants. In order to explore which of the two options may occur more frequently we started a project focused on interactions with animals and phenotypic and genetic differentiation among populations of D. laureola across the Baetic Ranges. This region in Southern Iberian Peninsula represents a glacial refugium and biodiversity hotspot in the western Mediterranean Basin. First, a spatially explicit analysis conducted on the vast majority of the species’ known populations in the study area showed that marginal populations (western and eastern) present larger spatial isolation than core populations, but are not smaller (read more). Furthermore, genetic analyses based on AFLP showed that core and marginal populations did not differ in genetic diversity, probably because of the occurrence of large populations on the local margins. However, whereas eastern populations constituted a genetically homogeneous group closely related to core populations, western populations were strongly differentiated from all the other populations and exhibited larger spatial and genetic differentiation among them (see here). As regards pollinators, we further found a shift in the main insect pollinator at the western margin (read more) where plants were not visited by the pollen beetle most commonly found on the core regions (see here). Regarding the consequences of this shift, we found that pollinators deposited larger pollen loads on the stigmas of individuals of marginal populations, contrary to the expectation of less efficient pollinator service on populations at distribution margins. Furthermore, hermaphrodites and females received similar quality pollen in marginal populations contrasting with core populations, where females received higher quality pollen than hermaphrodites (A. Castilla, work in progress).
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