Flowers naturally reproduce within their own species via genetic isolation mechanisms.
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Peer-reviewed literature demonstrates that flowering plants use diverse genetic and molecular mechanisms, such as peptide signals favoring conspecific pollen and internal hybridization barriers, to reproduce within their own species.
Racing to fertilization Pollen tubes, which carry plant sperm, need to grow from where they land in the flower to where the ovule is. Zhong et al. now show how pollen from related plant species race to reach the ovule first. One set of fast-evolving peptide signals is tuned to speed up growth of conspecific pollen tubes. A related set of evolutionarily ancient peptides is tuned to attract all pollen tubes. Thus, fertilization is more likely to happen through conspecific pollen tubes, but a fail-safe system encourages even the laggards to get where they need to go. Science, this issue p. eaau9564 Competition between fast- and slow-growing pollen tubes driven by peptide signals favors conspecific fertilization. INTRODUCTION Reproductive isolation is the inability of a species to breed with a related species and thus is key to the formation and evolution of a new plant species. Within the genus Arabidopsis, pistils of A. thaliana can be fertilized by pollen of related species, such as A. lyrata, but conspecific (self) pollen is preferred to maintain reproductive isolation. The molecular mechanisms by which the plant’s own pollen tubes outperform heterospecific (alien) pollen tubes is largely unknown. In A. thaliana, maternal cysteine-rich peptides AtLURE1.1 to -1.5, secreted from the ovule, were reported to function through the male receptor PRK6 to mediate pollen tube attraction. RATIONALE Although AtLURE1s have been identified as species-specific pollen tube attractants in A. thaliana, down-regulation of AtLURE1 genes and knockout of their receptor PRK6 did not disturb fertilization and seed set, indicating that the biological functions of AtLURE1s are not fully understood. We initially aimed to fully knock out genes for AtLURE1.1 to -1.5, as well as additional related AtLURE1 genes, to understand their contribution to fertilization success and reproductive isolation. RESULTS In addition to the five reported AtLURE1 genes, we identified two further A. thaliana–specific AtLURE1 genes and generated a loss-of-function atlure1 null septuple mutant by knocking out the whole gene family. Although atlure1 null mutants, resembling prk6 receptor mutants, exhibited normal fertility, pollen tubes in atlure1 null pistils, like prk6 pollen tubes, displayed delayed emergence at the septum. These data collectively suggest that AtLURE1-PRK6 signaling accelerates pollen tubes to penetrate the septum and to grow toward ovules. We further tested the function of AtLURE1-PRK6 signaling in reproductive isolation by depositing alien A. lyrata pollen onto A. thaliana pistils and showed that A. lyrata pollen tubes emerged much more slowly out of the septum. However, the emergence of A. lyrata pollen tubes in A. thaliana pistils was not further impaired in atlure1 null pistils, suggesting that AtLURE1s promote reproductive isolation by accelerating their own pollen tubes, which thus outperform alien tubes. This hypothesis requires the existence of more general, genus- and/or family-specific ovular guidance molecules. Therefore, we next investigated the functions of four AtLURE1-related Brassicaceae-conserved cysteine-rich peptides named XIUQIU1 to -4. We found that XIUQIU peptides attract pollen tubes in a non–species-specific manner and independently of the PRK6 receptor. This finding suggests that evolutionarily ancient XIUQIUs function as general pollen tube attractants in the Brassicaceae. Finally, after combining atlure1 null with xiuqiu loss-of-function mutations, fertility in A. thaliana was reduced, supporting the biological importance of these cysteine-rich peptides in plant reproduction. CONCLUSION In A. thaliana, species-specific female AtLURE1 peptides and their male receptor PRK6 promote and maintain reproductive isolation by accelerating conspecific pollen tube growth to penetrate the septum. The AtLURE1-related cysteine-rich XIUQIU peptides are evolutionarily ancient and conserved attractants in the Brassicaceae and attract pollen tubes i
The endosperm is a developmental innovation of angiosperms that supports embryo growth and germination. Aside from this essential reproductive function, the endosperm fuels angiosperm evolution by rapidly establishing reproductive barriers between incipient species. Specifically, the endosperm prevents hybridization of newly formed polyploids with their non-polyploid progenitors, a phenomenon termed the triploid block. Furthermore, recently diverged diploid species are frequently reproductively isolated by endosperm-based hybridization barriers. Current genetic approaches have revealed a prominent role for epigenetic processes establishing these barriers. In particular, imprinted genes, which are expressed in a parent-of-origin-specific manner, underpin the interploidy barrier in the model species Arabidopsis. We will discuss the mechanisms establishing hybridization barriers in the endosperm, the driving forces for these barriers and their impact for angiosperm evolution. This article is part of the theme issue ‘How does epigenetics influence the course of evolution?’
Wild potatoes are native to the Americas, where they present very wide geographical and ecological distribution. Most are diploid, obligate out-crossers due to a multiallelic gametophytic self-incompatibility (S) locus that prevents self-fertilisation and crossing between individuals carrying identical S-alleles. They have two alternative modes of reproduction: sexual (by seeds) and asexual (by stolons and tubers), which provide, respectively, for genetic flexibility in changing environments and high fitness of adapted genotypes under stable conditions. Since the early twentieth century, their taxonomic classification has been mostly based on morphological phenotypes (Taxonomic Species Concept). More recently, attempts have been made to establish phylogenetic relationships, applying molecular tools in samples of populations (accessions) with a previously assigned specific category. However, neither the reproductive biology and breeding relations among spontaneous populations nor the morphological and genetic variability expected in obligate allogamous populations are considered when the taxonomic species concept is applied. In nature, wild potato populations are isolated through external and internal hybridisation barriers; the latter, which are genetically determined, can be either pre-zygotic (pollen-pistil incompatibility) or post-zygotic (abortion of embryo, endosperm or both tissues, sterility, and hybrid weakness and breakdown in segregating generations). The internal barriers, however, can be incomplete, providing opportunities for hybridisation and introgression within and between populations and ploidy levels in areas of overlap. The widespread occurrence of spontaneous hybrids in nature was recognised in the mid-twentieth century. Using genetic approaches, results have been obtained that provide strong support to the assertion that populations are at different stages of genetic divergence and are not at the end of the evolutionary process, as presupposed by the Taxonomic Species Concept. Furthermore, since wild potatoes have uniparental and biparental overlapping generations, the Biological Species Concept - developed for sexually reproducing biparental organisms - cannot be applied to them. In this paper, morphological, genetic, molecular and taxonomic studies in wild potato are reviewed, considering the genetic consequences of their reproductive biology, in an attempt to shed light on the species problem, because of its relevance in germplasm conservation and breeding.
In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity. Mendel’s seminal work was accomplished using the garden pea, Pisum sativum, to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time.
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