Extremely high proportions of male flowers and geographic variation in floral ratios within male figs of Ficus tikoua despite pollinators displaying active pollen collection

Abstract

Most plants are pollinated passively, but active pollination has evolved among insects that depend on ovule fertilization for larval development. Anther‐to‐ovule ratios (A/O ratios, a coarse indicator of pollen‐to‐ovule ratios) are strong indicators of pollination mode in fig trees and are consistent within most species. However, unusually high values and high variation of A/O ratios (0.096–10.0) were detected among male plants from 41 natural populations of Ficus tikoua in China. Higher proportions of male (staminate) flowers were associated with a change in their distribution within the figs, from circum‐ostiolar to scattered. Plants bearing figs with ostiolar or scattered male flowers were geographically separated, with scattered male flowers found mainly on the Yungui Plateau in the southwest of our sample area. The A/O ratios of most F. tikoua figs were indicative of passive pollination, but its Ceratosolen fig wasp pollinator actively loads pollen into its pollen pockets. Additional pollen was also carried on their body surface and pollinators emerging from scattered‐flower figs had more surface pollen. Large amounts of pollen grains on the insects' body surface are usually indicative of a passive pollinator. This is the first recorded case of an actively pollinated Ficus species producing large amounts of pollen. Overall high A/O ratios, particularly in some populations, in combination with actively pollinating pollinators, may reflect a response by the plant to insufficient quantities of pollen transported in the wasps’ pollen pockets, together with geographic variation in this pollen limitation. This suggests an unstable scenario that could lead to eventual loss of wasp active pollination behavior.

Sky islands as foci for divergence of fig trees and their pollinators in southwest China

“…Mature figs containing fig wasp adult offspring were removed and placed separately in fine‐mesh bags to let wasps emerge naturally. The figs were then cut in half and the distribution patterns of their male flowers were recorded as either ostiolar, scattered or intermediate (Deng et al, ). The fig wasps that emerged from the figs were stored in ethanol at 4°C.…”

“…Within its large genus, F. tikoua is exceptional in terms of the variation it exhibits in inflorescence composition. Deng et al () identified three types of male flower distributions inside its male figs. They contain either small numbers of male flowers concentrated around the ostioles (ostiolar type) or larger numbers of flowers scattered throughout the figs (scattered type).…”

“…They contain either small numbers of male flowers concentrated around the ostioles (ostiolar type) or larger numbers of flowers scattered throughout the figs (scattered type). A few individuals have an intermediate condition, with small numbers of scattered male flowers (intermediate type) (Deng et al, ). The presence of small numbers of male flowers in figs is associated with active pollination behaviour in which pollinator females collect pollen from their natal figs into thoracic pockets and then unload the pollen in the figs where they lay their eggs.…”

“…Active pollination might be supplemented by passive pollination in areas where many male flowers are present in the figs, depending on the behaviour of the pollinators. The known pollinator of F. tikoua is an undescribed species of Ceratosolen (Cruaud et al, ; Deng et al, ) . Almost all Ceratosolen species are believed to be active pollinators, because they possess the coxal combs and pollen pockets that are used to manipulate and store pollen (Deng et al, ; Kjellberg et al, ).…”

“…The known pollinator of F. tikoua is an undescribed species of Ceratosolen (Cruaud et al, ; Deng et al, ) . Almost all Ceratosolen species are believed to be active pollinators, because they possess the coxal combs and pollen pockets that are used to manipulate and store pollen (Deng et al, ; Kjellberg et al, ). One African Ceratosolen species has reduced pollen pockets and no longer pollinates its hosts (Compton et al, ), but the species associated with F. tikoua consistently has functional pollen pockets that contain pollen, suggesting it is an active pollinator, despite the observed variation in its host figs (Deng et al, ).…”