Floral dimorphism, pollination, and self‐fertilization in gynodioecious GERANIUM RICHARDSONII (Geraniaceae)

Abstract

The selective maintenance of gynodioecy depends on the relative fitness of the male-sterile (female) and hermaphroditic morphs. Females may compensate for their loss of male fitness by reallocating resources from male function (pollen production and pollinator attraction) to female function (seeds and fruits), thus increasing seed production. Females may also benefit from their inability to self-fertilize if selfing and inbreeding depression reduce seed quality in hermaphrodites. We investigated how differences in floral resource allocation (flower size) between female and hermaphroditic plants affect two measures of female reproductive success, pollinator visitation and pollen receipt, in gynodioecious populations of Geranium richardsonii in Colorado. Using emasculation treatments in natural populations, we further examined whether selfing by autogamy and geitonogamy comprises a significant proportion of pollen receipt by hermaphrodites. Flowers of female plants are significantly smaller than those of hermaphrodites. The reduction in allocation to pollinator-attracting structures (petals) is correlated with a significant reduction in pollinator visitation to female flowers in artificial arrays. The reduction in attractiveness is further manifested in significantly less pollen being deposited on the stigmas of female flowers in natural populations. Autogamy is rare in these protandrous flowers, and geitonogamy accounts for most of the difference in pollen receipt between hermaphrodites and females. Female success at receiving pollen was negatively frequency dependent on the relative frequency of females in populations. Thus, two of the prerequisites for the maintenance of females in gynodioecious populations, differences in resource allocation between floral morphs and high selfing rates in hermaphrodites, occur in G. richardsonii.

Disentangling the effects of mating systems and mutation rates on cytoplamic diversity in gynodioecious Silene nutans and dioecious Silene otites

“…More generally, our results complement and partly confirm conclusions drawn by studies that used other methodological approaches to investigate the evolutionary dynamics driving the evolution of gynodioecy and found variation in sex ratio among populations that fits expectations under balancing selection (for example, Dufay et al (2009) in Beta vulgaris) and empirical evidence for frequency-dependent individual reproductive success, that is a necessary condition for such dynamics to occur (for example, Graff (1999) in Sidalcea malviflora; Williams et al (2000) in Geranium richardsonii; McCauley and Brock (1998) and Miyake and Olson (2009) The non-gynodioecious sister species to which the nucleotide diversity of S. nutans was compared in this study is dioecious (with males and females). This reproductive system has evolved many times independently in flowering plants (reviewed by Renner and Ricklefs (1995)).…”

Prolonged stigma and flower lifespan in females of the gynodioecious plant Geranium sylvaticum

“…This difference was similar for naturally pollinated flowers, except that those flowered four days shorter. Other studies on naturally pollinated flowers also showed an extended period of stigma lifespan for females (Ashman and Stanton, 1991;Petterson, 1992), and it has been suggested (but not measured) for several other species, including Geranium (Shykoff, 1992;Williams et al, 2000;Rodriguez-Riano and Dafni, 2007;Molano-Flores and Faivre, 2015). Whereas these other studies may indicate that extended maximum stigma lifespan in females is a general phenomenon among protandrous gynodioecious species, the differences in stigma lifespan may also be explained by the higher visitation rates observed for hermaphrodites.…”

“…richardsonii (Williams et al, 2000), depends on whether flower visitation and/or pollination have an effect on the duration of the floral phases. Pollinators cannot only induce senescence, shortening the female phase, but by removal of pollen they can also induce the female phase in some protandrous species (Evanhoe and Galloway, 2002;Zhang et al, 2011).…”

“…In dioecious and monoecious plants female flowers generally remain open for a longer time than male flowers (Primack, 1985). For several protandrous gynodioecious plants it has been suggested that while overall flower lifespan may not differ between the sexes, female flowers become receptive earlier for pollen and have a longer stigma lifespan than hermaphrodite flowers (Shykoff, 1992;Williams et al, 2000;Rodriguez-Riano and Dafni, 2007;Molano-Flores and Faivre, 2015).…”

“…Female flowers start unfolding their stigma early after flower opening . Individual flower lifespan has not been measured, but for the closely related G. richardsonii it was suggested, but not measured, that female flowers have 2-3 days longer lifespan than hermaphrodites (Williams et al, 2000). Some G. sylvaticum plants produce female and hermaphrodite flowers, or flowers with a reduced number of anthers (flowers of intermediate sexual expression) (Putrament, 1962).…”