Sex-ratio variation versus interplant distances in the regulation of pollen deposition and seed production in dioecious Cirsium arvense (Asteraceae)¹This article is part of a Special Issue entitled “Pollination biology research in Canada: Perspectives on a mutualism at different scales”.

Abstract

Pollination success in dioecious plants is expected to be regulated by the ratio of female:male plants and the distances between plants of each sex. These factors have received considerable attention in studies of dioecious and other gender dimorphic plants, yet their effects have rarely been jointly considered. We documented sex ratios in 26 populations of Cirsium arvense (L.) Scop. (creeping thistle), a widely distributed clonal dioecious plant, and show that population sex ratios are generally female biased (∼60% of shoots were female, and 15 of 26 populations were female biased). We found clear evidence for a negative association between pollen loads and distances between females and males within populations. In contrast, and in spite of broad variation in sex ratios among populations (range: 0.11–0.99 female), we found no relationship between sex ratios and pollen receipt by females. However, both sex ratios and interplant distances were associated with female seed production: seed production declined with distances between females and males and increased with the proportion of male shoots. Our data suggest that female plants of C. arvense might often be pollen limited and that distances between individual females and males drive pollen limitation more strongly than the overall frequency of males within sites.

Lonesome plants: How isolation affects seed set of a threatened dioecious shrub

“…Due to the inability of some individuals to self‐pollinate (De Jong et al., 2005 ), unless they are capable of apomixis (Dupont, 2002 ), seed production in obligate outcrossing dioecious species often heavily relies on the spatial arrangement of mating partners. For example, the density and distance to male plants can be negatively associated with the amount of pollen carried by insects (House, 1993 ) and deposited on stigmas (Van Drunen & Dorken, 2012 ), as well as with fruit and seed production (De Jong et al., 2005 ; House, 1992 ; Van Drunen & Dorken, 2012 ). However, the degree to which distance‐dependent pollination limits seed production in sexual polymorphic species can vary among populations because the different sexes are not necessarily evenly distributed within populations (Castilla et al., 2012 ).…”

“…Additionally, the sex ratio within populations can interact with the spatial arrangement of individuals and influence seed production. For example, a high aggregation of female plants and a low number of male plants can lead to reduced seed set (Öster & Eriksson, 2007 ; Timerman & Barrett, 2020 ; Van Drunen & Dorken, 2012 ).…”