Pollinator-Mediated Gene Flow Connects Green Roof Populations Across the Urban Matrix: A Paternity Analysis of the Self-Compatible Forb Penstemon hirsutus

Kelly Ksiazek-Mikenas,Krissa A Skogen,Jeremie B Fant

doi:10.3389/fevo.2019.00299

Kelly Ksiazek-Mikenas, Krissa A Skogen + Show 1 more

Open Access

https://doi.org/10.3389/fevo.2019.00299

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Abstract

Gene flow between populations can help maintain genetic diversity and prevent inbreeding, which is especially important for small, fragmented habitats. Many plant species rely on pollinators to move pollen between populations. In urban areas, insufficient pollinator services may result in limited gene flow, which can have negative consequences such as genetic drift and inbreeding depression. Furthermore, restored populations that are established with few founders of low genetic diversity may have limited long-term population persistence. Here, we tested the hypotheses that populations of a self-compatible forb established on urban green roofs from nursery stock are genetically depauperate and that limited gene (pollen) flow between populations will result in increased inbreeding. We compared the neutral genetic diversity of Penstemon hirsutus, using nine microsatellite loci, between three green roof populations established from nursery stock and three natural populations. We also established ten experimental populations on green roofs and measured rates of outcrossing and inbreeding and identified the movement of pollen within and between roofs using a paternity analysis. We found that neutral genetic diversity of populations established from nursery stock was lower than that of natural populations, although the level of inbreeding was also lower on the green roofs. In our experimental populations, we found that the rates of outcrossing and inbreeding varied between the roof populations. Our results suggest that inbreeding may be correlated with cover of co-flowering species but not with any of the other measured site properties. The location of likely pollen donors suggested that on average, 75% of pollen was derived from plants within the population (including self) and 25% came from plants on different roofs. Our results document realized pollen movement within and between green roofs, demonstrating that these habitats provide important connectivity in a fragmented environment.

Highlights

Green spaces are increasingly valued for their role in providing habitat for the conservation of native plants and wildlife in urban and suburban areas (Goddard et al, 2009; Kowarik, 2011)
While many plant species naturally occur in heterogeneous landscapes, the intervening matrix in urban areas may make urban populations more isolated than they would be in the wild (Frankham et al, 2002; Johnson et al, 2015; Hejkal et al, 2017)
The combined effects of increased isolation and limited genetic diversity may restrict the ability of urban plant populations to respond to environmental change and to limit their contribution to larger-scale dynamics required for long-term persistence and survival (Reed and Frankham, 2003; Frankham, 2005; Van Rossum, 2009)

Summary

INTRODUCTION

Green spaces are increasingly valued for their role in providing habitat for the conservation of native plants and wildlife in urban and suburban areas (Goddard et al, 2009; Kowarik, 2011). The ability to self-fertilize can ameliorate the effects of pollinator and pollen limitation in the short-term but inbreeding can create long-term challenges to population persistence if fitness is reduced (i.e., inbreeding depression; Frankham et al, 2002; Keller and Waller, 2002) Due to their spatial isolation (both vertical and horizontal), infrequent distribution, and occurrence in densely urbanized areas, green roof systems are an ideal setting for assessing the ability of urban ecosystems to achieve larger biodiversity conservation goals. Outcrossing may be limited if pollinator abundance is low and movement (gene flow) between green roofs is infrequent (Colla et al, 2009; MacIvor and Lundholm, 2011; Tonietto et al, 2011; Ksiazek et al, 2012) For these reasons, self-compatible plant species may experience higher selfing rates in cities (Kalisz et al, 2004) and lower fitness on green roofs (Ksiazek et al, 2012) compared to those in natural areas. We established ten experimental populations on green roofs and used paternity assignment to assess outcrossing rates and patterns of gene flow within and between urban green roofs

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DISCUSSION