The mating game: pollination and fertilization in flowering plants

Recent work has revealed signaling molecules that control pollination, including small peptides that mediate pollen recognition and glycoproteins that support pollen tube growth. The polarized growth of pollen tubes requires a calcium-mediated signal cascade, and cues derived from the haploid and diploid ovule cells guide pollen tubes to the eggs.     

The evolution of ovule number and flower size in wind-pollinated plants

In angiosperms, ovules are "packaged" within individual flowers, and an optimal strategy should occur depending on pollination and resource conditions. In animal-pollinated species, wide variation in ovule number per flower occurs, and this contrasts with wind-pollinated plants, where most species possess uniovulate flowers. This pattern is usually explained as an adaptive response to low pollen receipt in wind-pollinated species. Here, we develop a phenotypic model for the evolution of ovule number per flower that incorporates the aerodynamics of pollen capture and a fixed resource pool for provisioning of flowers, ovules, and seeds. Our results challenge the prevailing explanation for the association between uniovulate flowers and wind pollination. We demonstrate that when flowers are small and inexpensive, as they are in wind-pollinated species, ovule number should be minimized and lower than the average number of pollen tubes per style, even under stochastic pollination and fertilization regimes. The model predicts that plants benefit from producing many small inexpensive flowers, even though some flowers capture too few pollen grains to fertilize their ovules. Wind-pollinated plants with numerous flowers distributed throughout the inflorescence, each with a single ovule or a few ovules, sample more of the airstream, and this should maximize pollen capture and seed production.     

Insects and plants in the pollination ecology of the boreal zone

Pollination systems in the boreal zone range from generalist to specialist, both entomologically and botanically. The relative importance of wind pollination, insect pollination, sexual separation between and within plants, and between flowers, hermaphroditism of flowers, and various breeding systems are related to plant growth form and habitat. The diversity and specializations of anthophilous insects parallel those in other biogeographic zones, but seem less developed. We suggest that this reflects the combined effects of evolutionary youth, severity of climate, restriction of symbiont ranges within those of their hosts, and the naturally frequent perturbations by fire or insect outbreaks in the zone, requiring faunal and floral vagility and constraining specialization in mutualism. Modern perturbations by logging and pesticides seem to be well buffered because of the relative openness of the ecosystem (compared to others), although damage has been documented. Insect pollination is as much a keystone process in the boreal forest as elsewhere, despite the immediate counter-impression given by the dominance of wind-pollinated conifers. Nevertheless, there are few studies, botanical or entomologicalin situ. The boreal system offers important opportunities in general and applied research in pollination ecology and synecology generally.     

Structure and evolution of the Fusarium mating type locus: new insights from the Gibberellafujikuroi complex

Mating type genes are central to sexual reproduction and compatibility in Ascomycete fungi. However the MAT loci experience unique evolutionary pressures that can result in rapid divergence and enhanced inter-specific gene-flow (lateral gene transfer). In this study, molecular evolution of MAT loci was considered using the genus Fusarium (Teleomorph: Gibberella) as a model. Both MAT1-1 and MAT1-2 idiomorphs from eleven species of the Gibberellafujikuroi species complex were sequenced. Molecular evolution of the MAT loci from these heterothallic (self-sterile) species was compared with that of the MAT loci from nine homothallic (self-fertile) species in the Fusariumgraminearum species complex. Although Fusarium has previously been thought to have the same complement of four MAT genes that are found in Neurospora, we found evidence of a novel gene, MAT1-2-3, that may be specific to the Hypocreales. All MAT genes share a similar set of cis-regulatory motifs, although homothallic species might have recruited novel regulatory elements, which could potentially facilitate alternate expression of MAT1-1-1 and MAT1-2-1. FusariumMAT loci displayed evidence consistent with historical lateral gene-flow. Most notably, the MAT1-1 idiomorph of Fusariumsacchari appears to be unrelated to those of other species in the G.fujikuroi complex. In general, FusariumMAT genes are highly divergent. Both positive selection and relaxed selective constraint could account for this phenomenon. However, the extent of both recombination and inter-specific gene-flow in the MAT locus also appears to affect the rate of divergence.     

Fungal entomopathogens: new insights on their ecology

An important mechanism for insect pest control should be the use of fungal entomopathogens. Even though these organisms have been studied for more than 100 y, their effective use in the field remains elusive. Recently, however, it has been discovered that many of these entomopathogenic fungi play additional roles in nature. They are endophytes, antagonists of plant pathogens, associates with the rhizosphere, and possibly even plant growth promoting agents. These findings indicate that the ecological role of these fungi in the environment is not fully understood and limits our ability to employ them successfully for pest management. In this paper, we review the recently discovered roles played by many entomopathogenic fungi and propose new research strategies focused on alternate uses for these fungi. It seems likely that these agents can be used in multiple roles in protecting plants from pests and diseases and at the same time promoting plant growth.     

Molecular insights into the evolution of crop plants

The domestication and improvement of crop plants have long fascinated evolutionary biologists, geneticists, and anthropologists. In recent years, the development of increasingly powerful molecular and statistical tools has reinvigorated this now fast-paced field of research. In this paper, we provide an overview of how such tools have been applied to the study of crop evolution. We also highlight lessons that have been learned in light of a few long-standing and interrelated hypotheses concerning the origins of crop plants and the nature of the genetic changes underlying their evolution. We conclude by discussing compelling evolutionary genomic approaches that make possible the efficient and unbiased identification of genes controlling crop-related traits and provide further insight into the actual timing of selection on particular genomic regions.     

On the ecology and evolution of annual plants in disturbed environments

This paper is concerned with the effect of disturbance on some crucial characteristics of annual plants. The theoretically optimal life-history traits that maximize individual fitness in disturbed environments are described and critically evaluated. It seems that none of them holds for all annual species.Self-pollination and especially seed polymorphism are considered important adaptations to life in unpredictable environments. The thesis is put forward that amphicarpic annuals, which exhibit both self-pollination and extreme seed polymorphism, are best adapted to life in hazardous habitats. The hypothetical course of the evolution of amphicarpy is demonstrated on the grounds of the comparison of contemporary annual species producing chasmogamous and cleistogamous flowers on a single individual.     

Hydrophilous pollination and breeding system evolution in seagrasses: a phylogenetic approach to the evolutionary ecology of the Cymodoceaceae

COX, P. A. & HUMPHRIES, C. J. 1992. Hydrophilous pollination and breeding system evolution in seagrasses: a phylogenetic approach to the evolutionary ecology of the Cymodoceaceae. A phylogenetic analysis of seagrasses of the Cymodoceaceae shows the Cymodoceaceae to be monophyletic and Posidoniaceae to be their sister group. Information on the pollination ecologies and breeding systems of the various genera of the Cymodoceaceae were plotted onto the consensus tree obtained for the group. From this analysis, it is suggested that the clade composed of the Zosteraceae, Posidoniaceae and Cymodoceaceae evolved from a freshwater hydrophilous ancestor that developed submarine pollination and filiform pollen in association with invasion of the marine environment. Dioecism and surface pollination appear to have evolved in the progenitor of the Cymodoceaceae, and hence the seagrasses of the Cymodoceaceae are dioecious due to common descent rather than to convergent evolutionary processes in extant genera.     

N-acetylglucosamine induces white-to-opaque switching and mating in Candida tropicalis, providing new insights into adaptation and fungal sexual evolution

Pathogenic fungi are capable of switching between different phenotypes, each of which has a different biological advantage. In the most prevalent human fungal pathogen, Candida albicans, phenotypic transitions not only improve its adaptation to a continuously changing host microenvironment but also regulate sexual mating. In this report, we show that Candida tropicalis, another important human opportunistic pathogen, undergoes reversible and heritable phenotypic switching, referred to as the "white-opaque" transition. Here we show that N-acetylglucosamine (GlcNAc), an inducer of white-to-opaque switching in C. albicans, promotes opaque-cell formation and mating and also inhibits filamentation in a number of natural C. tropicalis strains. Our results suggest that host chemical signals may facilitate this phenotypic switching and mating of C. tropicalis, which had been previously thought to reproduce asexually. Overexpression of the C. tropicalis WOR1 gene in C. albicans induces opaque-cell formation. Additionally, an intermediate phase between white and opaque was observed in C. tropicalis, indicating that the switching could be tristable.     

Mitosis, stature and evolution of plant mating systems: low-Phi and high-Phi plants

There is a long-recognized association in plants between small stature and selfing, and large stature and outcrossing. Inbreeding depression is central to several hypotheses for this association, but differences in the evolutionary dynamics of inbreeding depression associated with differences in stature are rarely considered. Here, we propose and test the Phi model of plant mating system evolution, which assumes that the per-generation mutation rate of a plant is a function of the number of mitoses (Phi) that occur from zygote to gamete, and predicts fundamental differences between low-Phi (small-statured) and high-Phi (large-statured) plants in the outcomes of the joint evolution of outcrossing rate and inbreeding depression. Using a large dataset of published population genetic studies of angiosperms and conifers, we compute fitted values of inbreeding depression and deleterious mutation rates for small- and large-statured plants. Consistent with our Phi model, we find that populations of small-statured plants exhibit a range of mating systems, significantly lower mutation rates, and intermediate inbreeding depression, while large-statured plants exhibit very high mutation rates and the maximum inbreeding depression of unity. These results indicate that (i) inbred progeny typically observed in large-statured plant populations are completely lost prior to maturity in nearly all populations; (ii) evolutionary shifts from outcrossing to selfing are generally not possible in large-statured species, rather, large-statured species are more likely to evolve mating systems that avoid selfing such as self-incompatibility and dioecy; (iii) destabilization of the mating system-high selfing rate with high-inbreeding depression-might be a common occurrence in large-statured species; and (iv) large-statured species in fragmented populations might be at higher risk of extinction than previously thought. Our results help to unify and simplify a large and diverse field of research, and serve to emphasize the importance that developmental and genetic constraints play in the evolution of plant mating systems.     

Spliceosomal introns: new insights into their evolution

A new genome-wide analysis of spliceosomal introns indicates massive loss and gain of introns has taken place in many eukaryotic lineages. Only a small subset of the analyzed introns was present in the common ancestor of plants, fungi, animals and Plasmodium.     

Sex change in plants: Old and new observations and new hypotheses

Summary Evidence is presented that individuals of a large number of dioecious and subdioecious plant species are able to alter their sexual state in response to changes in the ambient environment and/or changes in size or age. We suggest that lability of sexual expression probably has survival value where a significant portion of the females must otherwise bear the cost of fruit production in unfavorable environments. We demonstrate that in patchy environments of the proper scale and variability in quality, labile sexual expression will enhance an individual's genetic contribution to the next generation.     

Devonian monospecific assemblages: new insights into the ecology of reduced-oxygen depositional settings

Low-diversity fossil assemblages interpreted as representing dysaerobic communities are common in the Phanerozoic rock record, and those composed of a single species have particular utility for recognizing the lowest bottom-water oxygen levels. An unusually high-diversity of clades including three rhynchonelliform, two linguliform, and one bivalve species occur in monospecific assemblages within Middle and Upper Devonian black shales of New York State. These six taxa are interpreted to be adapted to extremely reduced bottom-water oxygen levels as inferred from detailed sedimentological data and their repeated monospecific occurrence; however these groups represent two distinct ecologies. Three of these taxa are restricted to sediments deposited under the lowest dysaerobic conditions, while the other three taxa, unlike other fossils characteristic of reduced-oxygen levels, also occur in and are even dominant in high-diversity assemblages. The rhynchonellid brachiopod Eumetabolotoechia multicostata is the most abundant taxon within these units and has a remarkable ecological range as dominant from the lowest dysaerobic zone to near-normal marine oxygen levels. These Devonian groups, when present in monospecific assemblages, have utility for characterizing the lowest dysaerobic zone where trace fossil assemblages, most commonly used to describe these low-oxygen depositional settings, are absent or poorly developed.