Natural selection on flower size in invasive Cytisus scoparius along an elevation gradient

Aims Established invasive plant species have more diverse gene pools than single populations in their home range.This genetic and often phenotypic variation allows for natural selection to act and produce rapid adaptations as an invasive species interacts with other members of the community.For an invasive flowering plant,interactions with pollinators may lead to adaptations to a heterogeneous pollinator community.The variation in the pollinator community in the invaded range is similar to variation in the native range,suggesting that adaptations may mirror patterns seen in the home range.In this study,we investigated variation in flower size in Cytisus scoparius,an invasive leguminous shrub,along an elevation gradient.Methods This study took place in the Capitol State Forest in Washington State.We measured flower widths and the proportion of visited flowers,using 10 plants per field site.Sites were located along an elevation gradient and were visited three times to measure phenological variation on a per-plant basis.Important Findings We saw positive selection for flower size,albeit without a distinct pattern of higher selection at higher elevations.The pattern of natural selection could be seen both in terms of pollinator visitation rates and in seed production.We also found that the largest contributor to changes in seed production was not elevation or flower size but management practices.

Pollinators exert positive selection on flower size on urban,but not on rural Scotch broom(Cytisus scoparius L.Link)

Aims Adaptive evolution of invasive species is both particularly exciting for the evolutionary biologist and worrisome for those interested in controlling or halting spread.Invasive species often have a dis-tinct timeline and well-recorded population expansion.As invaders encounter new environments,they undergo rapid adaptive evolu-tion.Our aim in this study was to measure variation of floral size in the invasive shrub Cytisus scoparius(Scotch broom)and measure natural selection by pollinators on that trait.Past research has found that this invasive plant is pollinator limited in Washington State and that declines in pollinator populations can contribute to local extinction in another invaded range(New Zealand).This plant is pollinated by both native and introduced species of bees,represent-ing a broad range of pollinator sizes.Cytisus scoparius has a flower structure that is highly conducive to studies on pollinator choice,even in the absence of direct pollinator observations.Methods We surveyed urban and rural sites in and around the city of Olympia in Washington State.Measuring banner width,we were able to show that flower size varies substantially between plants but mini-mally within plants.By measuring the proportion of flowers that were‘tripped’,we could determine pollinator visitation rates and thus determine the level of selection due to pollinator choice alone.Important Findings We found that C.scoparius is under natural selection by pollinators for increased flower size.However,such positive natural selection was only seen in urban populations although it was consistent across two flowering seasons.Rural populations of Scotch broom do not appear to be under selection on flower size.The natural selection by pollinators on broom flowers could result in adaptive evolution into a new pollination niche by an invading species.A higher level of variation in broom flowers seen here than seen in previous works in native regions suggests that C.scoparius may be highly diverse and primed for adaptive evolution.