POLLINATOR-MEDIATED REPRODUCTIVE ISOLATION AMONG DIOECIOUS FIG SPECIES (FICUS, MORACEAE)

Abstract

Seed development and seedling growth were largely comparable between conspecific and heterospecific crosses. Heterospecific pollinator fitness, however, was significantly less than that of conspecific pollinators. Heterospecific pollinators induced gall formation but offspring did not develop to maturity in the new host. Selection on pollinators maintaining host specificity appears to be an important mechanism of contemporary reproductive isolation among these taxa that could potentially influence their diversification. K E Y W O R D S :Ceratosolen, coevolution, hybridization, speciation.The observation that some closely related plants co-occur as distinct species, even in the presence of hybridization, has long motivated the study of reproductive isolating mechanisms (Stebbins 1950). Reproductive isolation in plants can involve prepollination mechanisms, such as phenological, mechanical, and ethological barriers, or postpollination mechanisms, such as pollen incompatibility, genetic incompatibility, or hybrid inferiority (Grant 1971). In animal-pollinated taxa, pollinators may act as agents of reproductive isolation by means of species-specific floral constancy in foraging behavior (Oyama et al. 2010). Additionally, trait mismatching in heterospecific visitation can prevent heterospecific pollen deposition or the collection of pollinator rewards (Kephart and Theiss 2004;Kay 2006).When reproduction of pollinators and host is interdependent, selection for specialization may be intense, especially where similar hosts occur in sympatry. Indeed, the evolution of extreme pollinator specialization is observed in cases where plants provide "nursery" rewards for pollinators (Ollerton 2006). A recent natural experiment on the obligate pollination mutualism between Joshua trees and yucca moths (Smith et al. 2009) revealed that phenotype matching plays a role in reproductive isolation of parapatric host tree varieties. In spite of hybridization between varieties in a Ficus is known not only for its obligate pollination mutualism but also for extreme species richness (>750 species), and diversity in sympatry. In centers of endemism, such as New Guinea, recent radiations of closely related species coexist in sympatry ( Methods STUDY SYSTEMFicus bernaysii King, Ficus congesta Roxb., Ficus hahliana Diels, Ficus hispidioides S. Moore, Ficus morobensis C.C. Berg, and Ficus pachyrrhachis K. Schum. and Lauterb. are members of dioecious Ficus subgenus Sycomorus section Sycocarpus. They comprise the majority of a clade estimated to have originated in New Guinea at least 15 million years ago and within which precise phylogenetic relationships are unresolved (Silvieus et al. 2008). These New Guinea endemic taxa often occur at high density in patches of secondary regrowth. They have abundant cauliflorous figs growing along the length of the trunk and are pollinated by Ceratosolen species (Wiebes 1963). Most fig species in this study system have uniquely associated pollinator species. For example, Ceratosolen notus pollinates F. conges...

Pollinator sharing and gene flow among closely related sympatric dioecious fig taxa

“…R. Soc. B 283: 20152963 shifts, as shown here and in previous studies [13,14,23,32,33], suggest hybridization and introgression among symmetric fig species should be a natural outcome of pollinator sharing.…”

“…The pollinator sharing ratio may be overestimated with wasps collected from receptive syconia if wasps have lower survival on 'non-typical' hosts, compared with the ratio calculated based on wasps emerging from syconia in the male flower phase [14]. The high pollinator sharing ratio within F. auriculata complex may be partially explained by this reason, as all pollinators of FA and half the pollinators of FO were collected from receptive syconia (electronic supplementary material, table S1).…”

“…The mechanism and degree of pollinator specificity is a critical aspect for understanding genetic diversification in flowering plants, particularly among sympatric plants, where this specificity is a primary source for reproductive isolation among plant taxa [9,11,14]. The floral volatile signature, which often consists of a mixture of volatile organic compounds, is believed to be the main mechanism for maintaining pollinator specificity in many obligate pollination systems [10,15].…”

Faster speciation of fig‐wasps than their host figs leads to decoupled speciation dynamics: Snapshots across the speciation continuum

“…Still, contrary to the expectation under strict cospeciation, where speciation events of interacting lineages occur at the same time (Huelsenbeck, Rannala, & Larget, ). Moe and Weiblen () and Yu et al () have proposed faster speciation rates for wasps than their host figs, suggesting that occasional breakdowns in the one‐to‐one matching might be inevitable. While each of these studies provides useful insights, the results are hard to compare, as they span different regions, taxa, and different phases of the cospeciation process.…”

A Breakdown of Obligate Mutualism on a Small Island: An Interspecific Hybridization between Closely Related Fig Species (Ficus pumila and Ficus thunbergii) in Western Japan

“…Although many plant groups experience frequent hybridization events during their evolution [13] [14], Ficus is usually considered an exception because of its specific pollination mode [2]. However, recent estimations from genetic data on six sympatric dioecious fig species in New Guinea suggested that 12% of fig populations included hybrids [15]. Conversely, the genetic identities of sympatric fig species were strictly maintained though fig-wasp species were shared among them [16].…”