The best of two worlds: ecology and evolution of ambophilous plants

Ambophily, the mixed mode of wind and insect pollination is still poorly understood, even though it has been known to science for over 130 years. While its presence has been repeatedly inferred, experimental data remain regrettably rare. No specific suite of morphological or ecological characteristics has yet been identified for ambophilous plants and their ecology and evolution remain uncertain. In this review we summarise and evaluate our current understanding of ambophily, primarily based on experimental studies. A total of 128 ambophilous species - including several agriculturally important crops - have been reported from most major habitat types worldwide, but this probably represents only a small subset of ambophilous species. Ambophilous species have evolved both from wind- and insect-pollinated ancestors, with insect-pollinated ancestors mostly representing pollination by small, generalist flower visitors. We compiled floral and reproductive traits for known ambophilous species and compared our results to traits of species pollinated either by wind or by small generalist insects only. Floral traits were found to be heterogeneous and strongly overlap especially with those of species pollinated by small generalist insects, which are also the prominent pollinator group for ambophilous plants. A few ambophilous species are only pollinated by specialised bees or beetles in addition to pollination by wind. The heterogeneity of floral traits and high similarity to generalist small insect-pollinated species lead us to conclude that ambophily is not a separate pollination syndrome but includes species belonging to different insect- as well as wind-pollination syndromes. Ambophily therefore should be regarded as a pollination mode. We found that a number of ecological factors promoted the evolution of ambophily, including avoidance of pollen limitation and self-pollination, spatial flower interference and population density. However, the individual ecological factors favouring the transition to ambophily vary among species depending on species distribution, habitat, population structure and reproductive system. Finally, a number of experimental studies in combination with observations of floral traits of living and fossil species and dated phylogenies may indicate evolutionary stability. In some clades ambophily has likely prevailed for millions of years, for example in the castanoid clade of the Fagaceae.

Sex change in kiwifruit (Actinidia chinensis Planch.): a developmental framework for the bisexual to unisexual floral transition

The developmental morphology of male and female kiwifruit flowers is tracked to delimit a framework of events to aid the study of divergence in floral gene expression. The transition from hermaphrodite to unisexual development of kiwifruit (Actinidia chinensis Planch) flowers has been reported previously, but differences in gene expression controlling sexual development for this species have not been associated with the major developmental changes occurring within pistils. We investigated the key stages in male and female flower development to define the point at which meristematic activities diverge in the two sexes. A combination of scanning electron microscopy and light microscopy was used to investigate pistil development from the earliest stages. We identified seven distinct stages characterized by differences in ovary size and shape, macrosporogenesis, ovule primordium development, anther locule lengthening, microspore wall thickening, and pollen degeneration. Sex differences were evident from the initial stage of development, with a laterally compacted gynoecium in male flowers. However, the key developmental stage, at which tissue differentiation clearly deviated between the two sexes, was stage 3, when flowers were 3.5 to 4.5 mm in length at approximately 10 d from initiation of stamen development. At this stage, male flowers lacked evident carpel meristem development as denoted by a lack of ovule primordium formation. Pollen degeneration in female flowers, probably driven by programmed cell death, occurred at the late stage 6, while the final stage 7 was represented by pollen release. As the seven developmental stages are associated with specific morphological differences, including flower size, the scheme suggested here can provide the required framework for the future study of gene expression during the regulation of flower development in this crop species.

Plant Microbiome and Its Link to Plant Health: Host Species, Organs and Pseudomonas syringae pv. actinidiae Infection Shaping Bacterial Phyllosphere Communities of Kiwifruit Plants

Pseudomonas syringae pv. actinidiae (Psa) is the causal agent of the bacterial canker, the most devastating disease of kiwifruit vines. Before entering the host tissues, this pathogen has an epiphytic growth phase on kiwifruit flowers and leaves, thus the ecological interactions within epiphytic bacterial community may greatly influence the onset of the infection process. The bacterial community associated to the two most important cultivated kiwifruit species, Actinidia chinensis and Actinidia deliciosa, was described both on flowers and leaves using Illumina massive parallel sequencing of the V3 and V4 variable regions of the 16S rRNA gene. In addition, the effect of plant infection by Psa on the epiphytic bacterial community structure and biodiversity was investigated. Psa infection affected the phyllosphere microbiome structures in both species, however, its impact was more pronounced on A. deliciosa leaves, where a drastic drop in microbial biodiversity was observed. Furthermore, we also showed that Psa was always present in syndemic association with Pseudomonas syringae pv. syringae and Pseudomonas viridiflava, two other kiwifruit pathogens, suggesting the establishment of a pathogenic consortium leading to a higher pathogenesis capacity. Finally, the analyses of the dynamics of bacterial populations provided useful information for the screening and selection of potential biocontrol agents against Psa.

A one-step organelle capture: gynogenetic kiwifruits with paternal chloroplasts

Androgenesis, the development of a haploid embryo from a male nucleus, has been shown to result in the instantaneous uncoupling of the transmission of the organelle and nuclear genomes (with the nuclear genome originating from the male parent only and the organelle genomes from the female parent). We report, for the first time, uncoupling resulting from gynogenesis, in Actinidia deliciosa (kiwifruit), a plant species known for its paternal mode of chloroplast inheritance. After pollen irradiation, transmission of nuclear genes from the pollen parent to the progeny was inhibited, but transmission of the chloroplast genome was not. This demonstrates that plastids can be discharged from the pollen tube into the egg with little or no concomitant transmission of paternal nuclear genes. Such events of opposite inheritance of the organelle and nuclear genomes must be very rare in nature and are unlikely to endanger the long-term stability of the association between the different genomes of the cell. However, they could lead to incongruences between organelle gene trees and species trees and may constitute an alternative to the hybridization/introgression scenario commonly invoked to account for such incongruences.

Sex determination in plants

The majority of flowering plants produce flowers that are "perfect." These flowers are both staminate (with stamens) and pistillate (with one or more carpels). In a small number of species, there is spatial separation of the sexual organs either as monoecy, where the male and female organs are carried on separate flowers on the same plant, or dioecy, where male and female flowers are carried on separate male (staminate) or female (pistillate) individuals. Sex determination systems in plants, leading to unisexuality as monoecy or dioecy, have evolved independently many times. In dioecious plant species, the point of divergence from the hermaphrodite pattern shows wide variation between species, implying that the genetic bases are very different. This review considers monoecious and dioecious flowering plants and focuses on the underlying genetic and molecular mechanisms. We propose that dioecy arises either from monoecy as an environmentally unstable system controlled by plant growth substances or from hermaphroditism where the underlying mechanisms are highly stable and control does not involve plant growth substances.