Structure of floral nectaries and comparison of reproductive and vestigial organs in the staminate and pistillate flowers of dioeciousSilene latifolia(Caryophyllaceae)

Abstract

Silene latifolia Poiret of Eurasia has established in North America, prompting this structural study of its mature unisexual buds and flowers. Floral nectaries, anther and stigma changes, and vestigial reproductive structures were studied using light and scanning electron microscopy. In staminate flowers, anthers dehisced before anthesis and >90% of their pollen was liberated within 36 h. Accumulated in the tubular calyx, nectar descended an anthophore from the stomatal-bearing nectary at the stamen bases. Nectary tissue surrounded the pistillode, a central filamentous organ lacking ovules but tipped by hairs resembling stigmatic papillae. In pistillate flowers, nectar flowed into an inflated calyx. The annular nectary had 10 regularly spaced, stomatal-lined craters and was continuous with the adaxial surfaces of the infertile antisepalous and epipetalous staminodes. Key elements of entomophilous pollination leading to successful sexual reproduction of this invasive species include secondary nectar presentation from disparate floral nectaries that, for pistillate flowers, also incorporate the staminodes; rapid pollen release from anthers; and elongation of papillae by tip growth that enhances each stigma’s receptive surface. Context is also provided for future studies of floral nectary development in this model dioecious species.

Development differs between independently evolved staminode whorls in the same flower

“…Staminodes have commonly been studied in hermaphroditic flowers, in which a fraction of the androecium evolves into infertile structures, but few studies have addressed the evolution of staminodes as they occur through the complete loss of staminate function in carpellate flowers (e.g., Tucker, 1988; Cane, 1993). Staminodes that arise during the transition from bisexual to unisexual flowers are often interpreted as intermediate structures that will be lost through evolutionary time (Walker‐Larsen and Harder, 2000; Diggle et al, 2011), but they can alternatively transition into co‐opted staminodes that have important functions to attract pollinators, such as producing nectar (Weberling, 1989; Nores et al, 2013; Anderson et al, 2019). Although staminode‐evolution studies often dismiss unisexual flowers (e.g., Walker‐Larsen and Harder, 2000), we stand to gain novel insight into floral evolution through the study of these overlooked structures in unisexual flowers.…”

“…In some species, such as Carica papaya L. (Caricaceae), the androecium is completely lost in the carpellate flowers, and the gynoecium is reduced to a nectary in the staminate flowers (Storey, 1969; Ronse De Craene and Smets, 1999), a condition that Mitchell and Diggle (2005) termed Type‐II flowers. In contrast, Type‐I flowers still have vestigial staminodes or pistillodes in mature flowers, for example in Silene L. (Caryophyllaceae) and Bauhinia L. (Fabaceae) (Tucker, 1988; Mitchell and Diggle, 2005; Diggle et al, 2011; Anderson et al, 2019). Carpellate Type‐I flowers have been overlooked as systems to study staminode evolution because the selective forces for staminodes in unisexual flowers are likely different than those in hermaphroditic flowers (Walker‐Larsen and Harder, 2000); however, Botnaru and Schenk (2019) emphasized that the selective forces that favor staminodes in hermaphroditic flowers across taxa are equally as diverse.…”

“…Staminodes that arise during the transition from bisexual to unisexual flowers are often interpreted as intermediate structures that will be lost through evolutionary time (Walker-Larsen and Harder, 2000;Diggle et al, 2011), but they can alternatively transition into co-opted staminodes that have important functions to attract pollinators, such as producing nectar (Weberling, 1989;Nores et al, 2013;Anderson et al, 2019). Although staminode-evolution studies often dismiss unisexual flowers (e.g., Walker-Larsen and Harder, 2000), we stand to gain novel insight into floral evolution through the study of these overlooked structures in unisexual flowers.…”