The partnership between fig trees and fig wasps is characterized by extreme specificity and high levels of behavioral and morphological co-adaptation, much of it linked to the unusual structure of the Ficus inflorescence – the fig. Prof. YANG Darong and his team of Xishuangbanna Tropical Botanical Garden (XTBG) have been focusing on the co-evolution of figs and fig wasps for many years. In a recent study conducted on Ficus hispida L., a small free-standing functionally dioecious tree widely distributed across SE Asia ,at the fig collection of XTBG (21°55′N, 101°15′E, at about 555 m above sea level), they used controlled experiments to examine the interplay between host floral longevity, ostiole permeability and body size in fig wasps.
They asked (1) Does the willingness of pollinators to enter figs vary with the length of time that a fig has waited to be pollinated (fig age)? (2) Does ease of passage through the ostiole change with fig age? (3) Does successful entry into the figs vary according to the size of the wasps? (4) Are age-related changes similar in male and female figs? And (5) Do pollinators that enter older figs produce fewer or smaller offspring, with different sex ratios? Those questions were examined using crops that were produced during three different seasons, because floral longevity varies according to ambient temperatures.
Their study found that pollinators took longer to find the ostioles of older figs, and longer to penetrate them. They also became increasingly unwilling to enter figs as they aged, and increasing numbers of the wasps became trapped in the ostiolar bracts. Larger individuals were particularly unwilling to enter older figs, resulting in older figs being pollinated by smaller wasps. On female trees, where figs produce only seeds, seed production declined rapidly with fig age. On male trees, the numbers and size of fig wasp offspring declined, and a higher proportion were male. Older male figs were harder to enter, especially for larger individuals, and offered poorer quality oviposition opportunities.