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

Jun-Yin Deng,Finn Kjellberg,Yan Chen,Lu-Shui Zhang,Fan Yang,Stephen G Compton,Rong-Hua Fu,Dai-Mei Hu

doi:10.1002/ece3.1926

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.

Highlights

Around 80% of all flowering plant species require the services of insects for their sexual reproduction (Ghazoul 2005; Biesmeijer et al 2006; Potts et al 2010), making pollination of flowers by insects central to the maintenance of terrestrial biodiversity (Kearns et al 1998; Biesmeijer et al 2006)
Ecology and Evolution published by John Wiley & Sons Ltd
Using a dioecious Chinese species, F. tikoua, we address here the following questions: (1) How large is the range in male flower numbers within male figs of F. tikoua, and does the distribution of male flowers within figs change with A/O ratio? (2) Is the variation in male flower numbers continuous or discontinuous? (3) Is there a geographic pattern to the distribution of this floral trait? (4) Are pollinators actively collecting pollen throughout the range of the plant, and are the pollen grains unusually small? and (5) what are the potential drivers for variation in male flower numbers within this species?

Summary

Introduction

Around 80% of all flowering plant species require the services of insects for their sexual reproduction (Ghazoul 2005; Biesmeijer et al 2006; Potts et al 2010), making pollination of flowers by insects central to the maintenance of terrestrial biodiversity (Kearns et al 1998; Biesmeijer et al 2006). This can generate selection pressures that result in the evolution and modification of traits related to both a plant’s female and male reproductive functions (Larson and Barrett 2000; Fenster et al 2004; Knight et al 2005) This may affect how resources are allocated to male and female function by affecting inflorescence design, but not in a way that will maximize the efficiency of reproduction: the ESS trade-off between male and female investment is a compromise in investment when marginal male and female function benefits become equal (Fishbein and Venable 1996).

Methods

Results

Conclusion