Phenological adaptations in Ficus tikoua exhibit convergence with unrelated extra-tropical fig trees

Asymmetric pre-growing season warming may jeopardize seed reproduction of the sand-stabilizing shrub Caragana microphylla

Our current understanding of the processes and mechanisms by which seasonal asymmetric warming affects seed reproduction in semiarid regions, which are essential in preserving the stability of both vegetation ecosystem structure and function, remains poorly understood. Here, we conducted a field warming experiment, including pre-growing season warming (W1), in-growing season warming (W2), and combined pre- and in-growing season warming (W3) treatments, to investigate the seed reproductive strategy of Caragana microphylla, an important sand-stabilizing shrub, from the perspective of reproductive phenology, reproductive effort, and reproductive success. Results show that the warming treatments advanced the initial stages of reproductive phenology, prolonged its duration, and decreased its synchrony (magnitude = W3 > W2 > W1). Additionally, flowering phenology was more sensitive to warming than podding phenology. The W1 treatment inclined seed reproduction towards the conservative strategy with low reproductive effort and success. The W3 treatment tended to increase seed reproductive effort and success. While the W2 treatment did not affect reproductive success, it did increase reproductive effort. Changes in reproductive phenology explained 20 % of the variation in reproductive effort and 38 % of the variation in reproductive success. However, these changes also directly hindered reproductive success (direct effect = -0.57) while indirectly promoting reproductive success (indirect effect = 0.27) by increasing reproductive efforts. Our results reveal that the seasonal asymmetry of warming altered the seed reproduction strategy of sand-stabilizing shrubs, with warmer winters and springs before the growing season decreasing seed fecundity.

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

The ancient interaction between figs (Ficus, Moraceae) and their pollinating fig wasps is an unusual example of a mutualism between plants and gall-inducing insects. This review intends to offer fresh perspectives into the relationship between figs and the diversity of gall-inducing sycophiles which inhabit their enclosed globular inflorescences that function as microcosms. Besides gall-inducing pollinators, fig inflorescences are also inhabited by other gall-inducing wasps. This review evaluates the state of current knowledge on gall-induction by fig wasps and exposes the many lacunae in this area. This review makes connections between fig and gall-inducing wasp traits, and suggests relatively unexplored research avenues. This manuscript calls for an integrated approach that incorporates such diverse fields as life-history theory, plant mate choice, wasp sexual selection and local mate competition, plant embryology as well as seed and fruit dispersal. It calls for collaboration between researchers such as plant developmental biologists, insect physiologists, chemical ecologists and sensory biologists to jointly solve the many valuable questions that can be addressed in community ecology, co-evolution and species interaction biology using the fig inflorescence microcosm, that is inhabited by gall-inducing mutualistic and parasitic wasps, as a model system.

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

The dynamics of populations and their divergence over time have shaped current levels of biodiversity and in the case of the "sky islands" of mountainous southwest (SW) China have resulted in an area of exceptional botanical diversity. Ficus tikoua is a prostrate fig tree subendemic to the area that displays unique intraspecific diversity, producing figs typical of different pollination modes in different parts of its range. By combining climate models, genetic variation in populations of the tree's obligate fig wasp pollinators and distributions of the different plant phenotypes, we examined how this unusual situation may have developed. We identified three genetically distinct groups of a single Ceratosolen pollinator species that have largely parapatric distributions. The complex topography of the region contributed to genetic divergence among the pollinators by facilitating geographical isolation and providing refugia. Migration along elevations in response to climate oscillations further enhanced genetic differentiation of the three pollinator groups. Their distributions loosely correspond to the distributions of the functionally significant morphological differences in the male figs of their host plants, but postglacial expansion of one group has not been matched by spread of its associated plant phenotype, possibly due to a major river barrier. The results highlight how interplay between the complex topography of the "sky island" complex and climate change has shaped intraspecies differentiation and relationships between the plant and its pollinator. Similar processes may explain the exceptional botanical diversity of SW China.

Latitudinal effects on phenology near the northern limit of figs in China

The interaction between pollinating wasps and figs is an obligate plant-insect mutualism, and the ca. 750 Ficus species are mainly tropical. Climatic constraints may limit species distributions through their phenology and this seems particularly likely for figs, where phenological mismatches can cause local extinction of the short-lived pollinators. We therefore compared the phenologies of Ficus altissima, F. racemosa and F. semicordata in tropical Xishuangbanna (21°55′N) and subtropical Liuku (25°50′N), SW China, to understand what factors limit fig distributions near their northern limits. All species produced synchronous crops of syconia in Xishuangbanna but production in Liuku was continuous, which may help maintain pollinator populations. However, in general, we found decreased fitness at the northern site: slower syconium development, so fewer crops each year; fewer seeds per syconium (two species); and fewer pollinators and more non-pollinators per syconium, so less pollen is dispersed. This is most easily explained by colder winters, although low humidities may also contribute, and suggests the northern limit is set by temperature constraints on reproductive phenology. If so, the warming predicted for future decades is expected to enhance the fitness of northern populations of figs and, in the longer term, allow them to shift their range limits northwards.

Topographic effect on the phenology of Ficus pedunculosa var. mearnsii (Mearns fig) in its northern boundary distribution, Taiwan

Mearns fig grows at the edge of coastal vegetation on uplifted coral reefs, its population and mutualistic-pollinators are susceptible to the influence of extreme weather. To determine the phenology of Mearns fig and the effects of various weather events under small-scale topographic differences, phenology was conducted for 3 years and 7 months. Results showed that Mearns figs had multiple leaf and fig productions year-round. Topographic effects caused population in Frog Rock Trail and Jialeshuei, which are less than 10 km away from each other, to exhibit different phenological patterns after experiencing severe weather events. Northeast monsoons led the Jialeshuei population to show low amounts of leaves and figs in winter and the phenological production was also susceptible to disturbances by typhoons in summer. Fig reproduction in such environment was disadvantageous to maintain pollinators. Besides, topographic complex in microhabitat of Frog Rock Trail protected some individuals from these same events thus safeguard population’s survival. The phenology of Mearns fig would respond to the weather events sensitively, which serve as references for estimating the mutualism system, and as indicators of climate change.

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

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.

Winter cropping in Ficus tinctoria: an alternative strategy

The many species of figs (Ficus, Moraceae) have evolved a variety of reproductive phenologies that ensure the survival of both the fig plants and their short-lived, species-specific, pollinating wasps. A phenological study of 28 male and 23 female plants of a dioecious hemiepiphytic fig, Ficus tinctoria, was conducted in Xishuangbanna, SW China at the northern margin of tropical SE Asia. In contrast to other figs of seasonal climates, which have a winter low in fig production, both sexes produced their major fig crops at the coldest time of the year. Male plants released pollinators during the period when most female trees were receptive and male syconia had a long wasp-producing (D) phase, which ensured high levels of pollination. Female crops ripened at the end of the dry season, when they attracted numerous frugivorous birds and dispersed seeds can germinate with the first reliable rains. Few syconia were produced by either sex during the rest of the year, but these were sufficient to maintain local pollinator populations. We suggest that this unique phenological strategy has evolved to maximize seed dispersal and establishment in this seasonal climate.