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Breeding system and factors limiting fruit production in the nectar-less orchid Broughtonia lindenii

Low fruit set values in most orchids (especially epiphytic and tropical species) are normally thought to be the consequence of pollination constraints and/or limited resources. In particular, pollination constraints are mainly modulated by pollinator visitation rates, pollinator visitation behavior (promoting crossing or selfing), the type and number of pollinia deposited on stigmas (in the case of orchids with subequal pollinia) and the amount of pollen loaded per inflorescence. In order to assess to what extent these factors can affect fruit set in particular orchid-pollinator systems, the repercussions of some of these aspects on reproduction of Broughtonia lindenii (Fig. 1 and 2) were examined in a coastal population in western Cuba. The study focused on plant breeding system, importance of the pollen load and the type of pollinia on subsequent fruit and seed, the limiting factors of seed production and the interaction with pollinators.

Fig.1. Characteristics of the orchid Broughtonia lindenii: (a) frontal view, (b) lateral view of dissected flower, (c) detail of flower with the lip retired, showing the cuniculus, (d) flower dissected and detail of the label showing the hairy lamella, (e) plant habit, (f) anther cap, (g) pollinarium, (h) male of the bee Melissodes leprieuri landing at a not previously visited flower, (i) leaving the flower once it had removed and received the pollinarium on its thorax, (j) examining the cuniculus on a new previously visited flower after deposit the pollinia on stigma.

This species presents long-lasting flowers that senesce after all forms of effective visits of the pollinator. Within Orchidaceae, B. lindenii is the first report of pollination by bees of the Neotropical genus Melissodes, particularly males (Fig. 1 h-j and 2 b). Pollinator dependence for fruit production was demonstrated, while hand-pollination experiments revealed self-compatibility and inbreeding depression at seed level. Even when inbreeding affects a certain number of seeds per fruit, this mechanism could permit pollination success by maintaining a mixed breeding system in species exposed to very rare pollinator activity, such as B. lindenii it should be noticed.

Fig.2. Details of the flower of Brougthonia lindenii (a) and its pollinator Melissodes leprieurii (b). The arrow point to the pollinia attached to the thorax and the white spot used to identified the males in this bees species.

More pollinia on stigmas enhance the proportion of well-developed seeds. In contrast, the pollinia type used in pollination is not important for seed quality of fruits, suggesting that small pollinia are not rudimentary (Fig. 1 f and g).

Natural fruit set in two consecutive years was substantially affected by pollinator activity, and also by systematic predation by ants and caterpillars (Fig. 3). Considering that this orchid completely lacks nectar and that the local assemblage of pollinators and predators influence strongly its reproduction, a minor importance of resource constraints in this epiphyte (with long-lasting reserve structures) is confirmed at least for a short time, as previously reported in many other tropical rewardless epiphytic orchids.

The study of these aspects in the sister, endangered species B. cubensis and B. negrilensis, and in other taxa of the Broughtonia Alliance could be particularly relevant for its conservation, and for the potential novelty in explaining the evolution of pollination ecology of this group as a whole.

Fig.3. Patterns of loosing of reproductive potential in the population of Broughtonia lindenii at Canasí, during reproductive periods of 2005/2006. The arrows show the steps in which the most important loosing of its potential was detected for both years.

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