CONICET | Buscador de Institutos y Recursos Humanos

Abstract: Oil-flower species are highly specialized attracting a narrowgroup of oil-collecting bees in contrast to nectar-flower species visitedby a wide diversity of pollinators. The vast majority of the oil specieslack nectar as a pollinator reward; this suggests that the ancestors ofplants with oil flowers had either nectar-less flowers or faced strongselection against the production of nectar once oil secretion evolved.Monttea aphylla is one of the few species that simultaneously offers oiland nectar in the same flower to pollinators. In particular, we studiedphenotypic selection in 16 populations in the Monte desert of Argentinathat spanned the entire geographical range of the species. In eachpopulation, we determined the relationship between plant pollinationsuccess and floral rewards. Positive directional selection differentialswere detected in three of the populations. One northern populationfavoured selection on oil production, two central or southern populationsfavoured selection on nectar production. Directional selection gradientswere consistent with the above results and correlational selection wassignificant for one northern population where pollination success wasfavoured by an increased selection on oil and a decreased selection onnectar. Geographic variation in phenotypic selection models showedlatitudinal increase in directional selection on nectar. Geographicvariation in visitation frequencies of the specialist oil-collecting beeand relative abundance of other floral sources were related to anincrease in directional selection on nectar. Thus, evolution of rewardsappears to respond to a geographic mosaic where either oil or nectar arefavoured in different contexts of ecologically specialised oil collectingbees. Although phenotypic selection was detectable in few and was absentin most populations, the general pattern was consistent with currentgeographic differentiation in reward amounts.

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