The synstigma turns the fig into a large flower

Abstract

The synstigma is a structure formed by clusters of two to several stigmas, whether in the same or between different flowers. Although rare in angiosperms, synstigmas are found in c. 500 out of the c. 750 Ficus spp. (Moraceae). This floral structure is associated with fig-fig wasp pollinating mutualism. The synstigma structure and pollen tube pathways were studied in six Ficus spp. from Ficus section Americanae to test the hypothesis that the synstigma allows pollen grains deposited on a stigma to emit pollen tubes that can grow laterally and fertilize surrounding flowers. Syconia containing recently pollinated stigmas were collected and dissected, and the stigmas were processed for analyses with light and scanning and transmission electron microscopy. The arrangement of the synstigmas across species can be spaced or congested, with the number of stigmas per synstigma ranging from two to 20. Contact between the stigmas in a synstigma occurs by the intertwining of the stigmatic branches and papillae; their union is firm or loose. The pollen tube grows through live cells of the transmitting tissue until reaching the ovule micropyle. Curved pollen tubes growing from one stigma to another were observed in five out of the six species studied. The curvilinear morphology of pollen tubes probably results from competition by pollen between the stigmas composing a synstigma via chemotropic signals. The synstigma appears to be a key adaptation that ensures seed production by flowers not exploited by the fig wasps in actively pollinated Ficus spp.

Pollinator sharing and hybridization in a pair of dioecious figs sheds light on the pathways to speciation

“…This absence could be attributed to the well-developed synstigma, a structure by the clustering of two or more stigmas ( Verkerke, 1989 ). The synstigma plays a pivotal role in ensuring seed production ( Teixeira et al, 2021 ), while concurrently presenting a challenge for wasps in locating suitable oviposition sites ( Shi et al, 2006 ). Thus, the length of the style itself and the structure of the synstigma may potentially impose constraints on wasp oviposition within female figs.…”

Interactions Between Figs and Gall-Inducing Fig Wasps: Adaptations, Constraints, and Unanswered Questions

“…Monoecious figs have imperfect heterostyly, i.e., flowers are arranged in multiple layers with style lengths that are either normally distributed or have a positively skewed distribution; flower pedicel lengths often vary such that all stigmas reach the same level (Kathuria et al, 1995;Ganeshaiah et al, 1999) to form a fused synstigma by the coherence of groups of stigmas (Teixeira et al, 2021; Figure 2). The synstigma is typical of actively pollinated fig species (see later).…”

“…Dioecious figs have perfect heterostyly; syconia on male trees have short-styled flowers, while those on female trees are long-styled (Figure 1). The synstigma allows pollen deposited at one location within this fused network to grow down a distant style at another location (Teixeira et al, 2021; Figure 2). This is analogous to the hyperstigma of Tambourissa (Monimiaceae) in which flowers are located within an invaginated cup (reminiscent of a syconium) closed by a mucilaginous hyperstigma (Endress, 1979).…”

“…Synstigmas occur in 67% of figs, more often in monoecious and actively pollinated species (Teixeira et al, 2018(Teixeira et al, , 2021. Stigmas of long-styled flowers in dioecious and monoecious species have greater pollen capture area compared to short-styled flowers (Jousselin et al, 2004;Teixeira et al, 2018).…”

“…How mate choice may be exercised in those fig species possessing synstigmas is enigmatic and completely unknown. The synstigma forms a compitum (transmission tract) for pollen tube competition which may also contribute to male reproductive success (Teixeira et al, 2021). We, therefore, need good plant embryology coupled with egg deposition and larval development, and also good insect oviposition behavior observations as in older classic papers (e.g., Johri and Konar, 1955;Galil and Snitzer-Pasternak, 1970;Galil and Eisikowitch, 1974).…”