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The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the ‘selfing syndrome’, in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Premise of Study In a seminal body of theory, Lloyd showed that the fitness consequences of selfing will depend on its timing in anthesis. Selfing that occurs after opportunities for outcrossing or pollen dispersal can provide reproductive assurance when pollinators are limited and is expected to incur little cost, even when inbreeding depression is high. As a result, delayed selfing is often interpreted as a “best‐of‐both‐worlds” mating system that combines the advantages of selfing and outcrossing. Methods We surveyed 65 empirical studies of delayed selfing, recording floral mechanisms and examining information on inbreeding depression, autofertility, and other parameters to test the support for delayed selfing as a best‐of‐both‐worlds strategy. Key Results Phylogenetic distribution of the diverse floral mechanisms suggests that some basic floral structures may predispose plant taxa to evolve delayed selfing. Delayed selfing appears to serve as a best‐of‐both‐worlds strategy in some but not all species. While the capacity for autonomous selfing is often high, it is lower, in some cases, than in related species with earlier modes of selfing. In other delayed‐selfers, low inbreeding depression and reduced investment in corollas and pollen suggest limited benefits from outcrossing. Conclusions Despite a growing literature on the subject, experimental evidence for delayed selfing is limited and major gaps in knowledge remain, particularly with respect to the stability of delayed selfing and the conditions that may favor transitions between delayed and earlier selfing. Finally, we suggest a potential role of delayed selfing in facilitating transitions from self‐incompatibility to selfing.

There is an ongoing debate about whether simultaneous hermaphrodites capable of selfing should prefer selfing over outcrossing or vice versa. While many theoretical models predict a transmission advantage for alleles that favour selfing, empirical studies often reveal low selfing rates. Despite these considerations, the underlying mechanisms that determine reproductive strategies in simultaneously hermaphroditic animals are poorly understood. In our study on the facultatively selfing free-living flatworm, Macrostomum hystrix, we ask whether the initiation of selfing, as inferred from the differential spatial distribution of received sperm, is linked to an individual’s female or male reproductive function. Specifically, the initiation of selfing could (i) be linked to the male function, when an individual is unable to donate sperm to others and hence donates sperm to self, or it could (ii) be linked to the female function, when an individual fails to receive sperm from others—and hence is unable to fertilize its eggs via outcrossing—thus inducing it to self-fertilize. We experimentally created a social environment that allowed focals to outcross via sperm donation, but simultaneously prevented them from receiving sperm—by pairing them with a partner lacking the male copulatory organ—so that fertilization of the focal’s eggs was restricted to selfing. Our results suggest that such focals generally do not initiate selfing, while we readily observe selfing in isolated worms. This suggests that in isolated M. hystrix, it is the male function that is linked to the initiation of selfing, likely due to a lack of opportunities for sperm donation.

• Background and Aims Reproductive assurance through autonomous selfing is thought to be one of the main advantages of self-fertilization in plants. Floral mechanisms that ensure autonomous seed set are therefore more likely to occur in species that grow in habitats where pollination is scarce and/or unpredictable. • Methods Emasculation and pollen supplementation experiments were conducted under laboratory conditions to investigate the capacity for, and timing of autonomous selfing in three closely related Centaurium species (Centaurium erythraea, C. littorale and C. pulchellum). In addition, observations of flower visitors were combined with emasculation and pollen addition experiments in natural populations to investigate the degree of pollinator limitation and pollination failure and to assess the extent to which autonomous selfing conferred reproductive assurance. • Results All three species were capable of autonomous selfing, although this capacity differed significantly between species (index of autonomous selfing 0·55 ± 0·06, 0·68 ± 0·09 and 0·92 ± 0·03 for C. erythraea, C. littorale and C. pulchellum, respectively). The efficiency and timing of autogamous selfing was primarily associated with differences in the degree of herkogamy and dichogamy. The number of floral visitors showed significant interspecific differences, with 1·6 ± 0·6, 5·4 ± 0·6 and 14·5 ± 2·1 floral visitors within a 2 × 2 m² plot per 20-min observation period, for C. pulchellum, C. littorale and C. erythraea, respectively. Concomitantly, pollinator failure was highest in C. pulchellum and lowest in C. erythraea. Nonetheless, all three study species showed very low levels of pollen limitation (index of pollen limitation 0·14 ± 0·03, 0·11 ± 0·03 and 0·09 ± 0·02 for C. erythraea, C. littorale and C. pulchellum, respectively), indicating that autonomous selfing may guarantee reproductive assurance. • Conclusions These findings show that limited availability of pollinators may select for floral traits and plant mating strategies that lead to a system of reproductive assurance via autonomous selfing.

Pollinator unpredictability favors evolutionary shifts from outcrossing to autonomous selfing, which provides reproductive assurance. Our goal was to quantify the reproductive assurance benefit of autonomous selfing and the stochastic nature of pollinatormediated pollen receipt using three wild populations of the annual species Collinsia verna (Scrophulariaceae) over three years. The timing of autonomous self-pollination in C. verna ranges from competing to delayed, with more than half of the autonomous pollen arriving late in floral life. Floral density, identity of insect floral visitors, and visitation rate varied within and among years, with significant variation in pollinator community between years within populations. Pollinator failure reduced fruit set within and among populations across years. However, reproductive assurance selfing increased fruit set on average by 8% overall and up to 30% in one population. Despite this fitness benefit, fruit set (= Σfruits/Σflowers) estimated in pollinator-exclusion cages was significantly less than the fruit set in paired open-pollinated plots. In a series of experiments, we found no difference in fruit set from self- vs. outcross pollen, ruling out early inbreeding depression. However, flowers, hand-outcrossed at the same time that autonomous selfing naturally occurs, produced significantly more fruits relative to flowers that received pollen only through autonomous selfing. These results suggest that pollen type (self/outcross) is less important to fruiting success than is the timing or efficiency of autonomous pollination relative to vector-mediated pollination. Our results support the reproductive assurance hypothesis and suggest that autonomous selfing in C. verna is adaptive under variable pollinator conditions. Fruit set results from both pollinator-delivered and autonomous self-pollen, suggesting a \"best of both worlds\" scenario, with mixed mating a likely outcome.

Background and Aims How and why plants evolve to become selfing is a long-standing evolutionary puzzle. The transition from outcrossing to highly selfing is less well understood in self-compatible (SC) mixed-mating (MM) species where potentially subtle interactions between floral phenotypes and the environment are at play. We examined floral morphological and developmental traits across an entire SC MM genus, Collinsia, to determine which, if any, predict potential autonomous selfing ability when pollinators are absent (AS) and actual selfing rates in the wild, s m, and to best define the selfing syndrome for this clade. Methods Using polymorphic microsatellite markers, we obtained 30 population-level estimates of s m across 19 Collinsia taxa. Species grand means for the timing of herkogamy (stigma-anther contact) and dichogamy (stigmatic receptivity, SR), AS, floral size, longevity and their genetic correlations were quantified for 22 taxa. Key Results Species fell into discrete selfing and outcrossing groups based on floral traits. Loss of dichogamy defines Collinsia's selfing syndrome. Floral size, longevity and herkogamy also differ significantly between these groups. Most taxa have high AS rates (>80 %), but AS is uncorrelated with any measured trait. In contrast, s m is significantly correlated only with SR. High variance in s m was observed in the two groups. Conclusions Collinsia species exhibit clear morphological and developmental traits diagnostic of 'selfing' or 'outcrossing' groups. However, many species in both the 'selfing' and the 'outcrossing' groups were MM, pointing to the critical influence of the pollination environment, the timing of AS and outcross pollen prepotency on s m. Flower size is a poor predictor of Collinsia species' field selfing rates and this result may apply to many SC species. Assessment of the variation in the pollination environment, which can increase selfing rates in more 'outcrossing' species but can also decrease selfing rates in more 'selfing' species, is critical to understanding mating system evolution of SC MM taxa.

Key MessageIncreased efficiencies achieved in different steps of DH line production offer greater benefits to maize breeding programs.Doubled haploid (DH) technology has become an integral part of many commercial maize breeding programs as DH lines offer several economic, logistic and genetic benefits over conventional inbred lines. Further, new advances in DH technology continue to improve the efficiency of DH line development and fuel its increased adoption in breeding programs worldwide. The established method for maize DH production covered in this review involves in vivo induction of maternal haploids by a male haploid inducer genotype, identification of haploids from diploids at the seed or seedling stage, chromosome doubling of haploid (D0) seedlings and finally, selfing of fertile D0 plants. Development of haploid inducers with high haploid induction rates and adaptation to different target environments have facilitated increased adoption of DH technology in the tropics. New marker systems for haploid identification, such as the red root marker and high oil marker, are being increasingly integrated into new haploid inducers and have the potential to make DH technology accessible in germplasm such as some Flint, landrace, or tropical material, where the standard R1-nj marker is inhibited. Automation holds great promise to further reduce the cost and time in haploid identification. Increasing success rates in chromosome doubling protocols and/or reducing environmental and human toxicity of chromosome doubling protocols, including research on genetic improvement in spontaneous chromosome doubling, have the potential to greatly reduce the production costs per DH line.

The evolution from outcrossing to predominant self-fertilization represents one of the most common transitions in flowering plant evolution. This shift in mating system is almost universally associated with the \"selfing syndrome,\" characterized by marked reduction in flower size and a breakdown of the morphological and genetic mechanisms that prevent self-fertilization. In general, the timescale in which these transitions occur, and the evolutionary dynamics associated with the evolution of the selfing syndrome are poorly known. We investigated the origin and evolution of selfing in the annual plant Capsella rubella from its self-incompatible, outcrossing progenitor Capsella grandiflora by characterizing multilocus patterns of DNA sequence variation at nuclear genes. We estimate that the transition to selfing and subsequent geographic expansion have taken place during the past 20,000 years. This transition was probably associated with a shift from stable equilibrium toward a near-complete population bottleneck causing a major reduction in effective population size. The timing and severe founder event support the hypothesis that selfing was favored during colonization as new habitats emerged after the last glaciation and the expansion of agriculture. These results suggest that natural selection for reproductive assurance can lead to major morphological evolution and speciation on relatively short evolutionary timescales.

Background and AimsThe establishment of plant populations in novel environments may generate pronounced shifts in floral traits and plant mating systems, particularly when pollinators are scarce. In this study, floral morphology and mating system functioning are compared between recently established and older populations of the annual plant Blackstonia perfoliata that occur in different pollinator environments.MethodsHand-pollination and emasculation experiments were conducted to assess the extent of pollinator-mediated pollen deposition and pollen limitation, and the contribution of autonomous selfing to total seed production. Detailed measurements of key floral traits were performed to compare the flower morphology and mating system functioning between plants from both pollination environments.Key ResultsPollinator-mediated pollen deposition was about twice as low in the recently colonized and pollinator-poor environment compared with the old and pollinator-rich sites, but total seed set was little affected by any type of pollen limitation. The contribution of autonomous selfing to total seed production was higher in the pollinator-poor sites than in the pollinator-rich sites (index of reproductive assurance = 0·56 and 0·17, respectively), and seed production was only poorly affected by selfing, whereas in the pollinator-rich populations selfing reduced total reproductive output by about 40 % compared with outcross pollination. Plants originating from pollinator-poor environments produced smaller flowers that showed significantly lower levels of dichogamy (i.e. protogyny) and herkogamy. These reductions resulted in a 2-fold higher capacity for autonomous selfing under pollinator-free conditions (index of autonomous selfing = 0·81 and 0·41 in plants originating from the pollinator-poor and pollinator-rich environment, respectively).ConclusionsThe results illustrate that plant populations colonizing novel environments can differ markedly in floral morphology and mating system functioning. Due to a temporal shift in the male phase, the breeding system of B. perfoliata shifted from delayed selfing under pollinator-rich conditions towards competing selfing in recently established populations, providing greater reproductive assurance when pollinators and/or reproductive partners are limited.

1. In plants that are able to self-fertilize, autonomous selfing has been hypothesized to function as an effective mechanism that prevents heterospecific mating. However, there have only been few studies that have assessed the relative and absolute contribution of different modes of selfing to total reproductive isolation acting between species. Because selfing can be expected to increase fitness costs when offspring are sired from more outcrossing sister taxa, it can be hypothesized that the contribution of post-zygotic barriers to total reproductive isolation becomes stronger in selfing than in more outcrossing relatives. 2. To assess the efficiency of different modes of selfing against heterospecific mating, we conducted mixed-pollination experiments in three Centaurium species showing strong differences in mating system (i.e. delayed selfing in Centaurium erythraea, competing selfing in C. littorale and prior selfing in C. pulchellum). The contribution of selfing to total reproductive isolation was further examined by quantifying the strength of three pre-zygotic (flowering phenology, pollinator fidelity and pollen production) and four intrinsic post-zygotic (seed set and germination, survival and flowering capacity) barriers for each of the six species pair combinations. 3. Although autonomous selfing as a reproductive barrier was unable to completely impede gene flow, its efficiency as a mechanism to prevents heterospecific pollination strongly depended on the mode of selfing, with prior selfing being by far the most efficient mechanism and delayed selfing only offering a limited protection against hybrid fertilization at the end of a flower's lifetime. Apart from the prevention effect of selfing, differences in pollinator fidelity and pollen production also contributed substantially to total reproductive isolation. 4. Post-zygotic hybrid costs were high and most prevalent for seed production and germination success. Reductions in hybrid vigour were significantly associated with the mating system of the maternal species, with the highest reductions in the most selfing C. pulchellum and the lowest reductions in predominant outcrossing C. erythraea. 5. Synthesis. Our findings indicate that selfing may prevent species from interspecific gene flow and hybridization. The association between the capacity to self-autonomously and hybrid vigour further suggests that selfing may foster post-zygotic costs following hybridization.