Search

Empirical description of phenotypic shifts in selection experiments have nourished quantitative genetics models. These models have shown that long-term selection responses depend both on the initial standing variance as well as the flux of incoming mutations, and that quantifying the proportion of genetic variance due to latter is essential to understanding the seeming absence of selection limits. There are however at least two important things to keep in mind when interpreting selection experiments: first correlative responses to selection on non-focal traits can perturb the selection response on the focal trait and second, generations and selection environments are confounded so that genotype by environment interactions are ignored. Here we used the Saclay Divergent Selection Experiments (DSE) on maize flowering time that display striking selection responses over 22 generations and combined yearly measurements of flowering time (DSEYM), measurements of 11 traits on a subset of 308 DSE genotypes measured for two consecutive years in common gardens (DSECG), as well as genotyping data to track the fate of individual mutations in pedigrees, with four main objectives: (1) to quantify the relative contribution of standing variation andde novomutational variance to the selection response; (2) to approximate the distribution of phenotypic effects of de novo mutations; (3) to study the impact of GxE interactions in the observed selection response; (4) to describe how trait correlations modulate the exploration of the phenotypic space. In agreement with theoretical predictions, we experimentally validated two successive phases in the selection response: one governed by the fixation of standing variants whose contribution quickly reached a plateau, and the second governed by the fixation of de novo mutations sustaining the selection response. The distribution of selected phenotypic effects confirmed the expected enrichment of fixation of beneficial mutations with an average effect of +0.278 and +0.299 days to flowering, depending on the genetic background; but also revealed fixation of unfavorable mutations reaching up to 25% of incoming mutations. Accumulation of genetic load may be due to antagonistic pleiotropy whereby mutations that were fixed in the selection environment (DSEYM) turn to be unfavorable in evaluation environment (DSECG). Finally, we found that global patterns of correlations between-traits are conserved across genetic backgrounds but exhibit a temporal pattern. Traits that are weakly or uncorrelated with flowering time allow stochastic exploration of the phenotypic space, owing to the microenvironment-specific fixation of standing variants and a pleiotropic mutational input.

The time between appearance of successive leaves or phyllochron enables to characterize the vegetative development of maize plants which determines their flowering time. Phyllochron is usually considered as constant over the development of a given plant, even though studies have demonstrated response of growth parameters to environmental variables. In this paper, we proposed a novel statistical approach for phyllochron analysis based on a stochastic process, which combines flexibility and a more accurate modelling than existing regression models. The model enables accurate estimation of the phyllochron associated with each leaf rank and enables hypothesis testing. We applied the model on an original maize dataset collected in fields from plants belonging to closely related genotypes originated from divergent selection experiments for flowering time conducted on two maize inbred lines. We showed that the main differences in phyllochron were not observed between selection populations (Early or Late), but rather ancestral lines, years of experimentation, and leaf ranks. Finally, we showed that phyllochron variations through seasons could be related to climate variations, even if the impact of each climatic variables individually was not clearly elucidated. All script and data can be found at \url{https://doi.org/10.15454/CUEHO6}

Background and aims Pollen grains of flowering plants display a fascinating diversity of forms, including diverse patterns of apertures, the specialized areas on the pollen surface that commonly serve as the sites of pollen tube initiation and, therefore, might play a key role in reproduction. Although many aperture patterns exist in angiosperms, pollen with three apertures (triaperturate) constitutes the predominant pollen type found in eudicot species. The aim of this study was to explore whether having three apertures provides selective advantages over other aperture patterns in terms of pollen survival, germination and reproductive success, which could potentially explain the prevalence of triaperturate pollen among eudicots. Methods The in vivo pollen germination, pollen tube growth, longevity and competitive ability to sire seeds were compared among pollen grains of Arabidopsis thaliana with different aperture numbers. For this, an arabidopsis pollen aperture series was used, which included the triaperturate wild type, as well as mutants without an aperture (inaperturate) and with more than three apertures. Key results Aperture number appears to influence pollen grain performance. In most germination and longevity experiments, the triaperturate and inaperturate pollen grains performed better than pollen with higher aperture numbers. In mixed pollinations, in which triaperturate and inaperturate pollen were forced to compete with each other, the triaperturate pollen outperformed the inaperturate pollen. Conclusions Triaperturate pollen grains might provide the best trade-off among various pollen performance traits, thus explaining the prevalence of this morphological trait in the eudicot clade.

International audience; Genome size evolution, and its relationships with pollen grain size, has been investigated in the sweet potato (Ipomoea batatas), an economically important crop, and closely related diploid and tetraploid species, assessing the nuclear DNA content of 22 accessions from five Ipomoea species, 10 sweet potato varieties and two outgroup taxa. Nuclear DNA amounts were determined by flow cytometry. Pollen grains have been studied at scanning and transmission electron microscopy. 2C DNA content of hexaploid I. batatas ranged over 3.12-3.29 pg, mean monoploid genome size being 0.539 pg (527 Mbp) much as for the related diploid accessions. In tetraploid species I. trifida and I. tabascana, 2C DNA content was respectively 2.07 and 2.03 pg. In the diploid species closely related to sweet potato e.g. I. ×leucantha, I. tiliacea, I. trifida, I. triloba, 2C DNA content was 1.01-1.12 pg. However, two diploid outgroup species, I. setosa and I. purpurea, were clearly different from the other diploid species with 2C of 1.47-1.49 pg; they also have larger chromosomes. The I. batatas genome presents 60.0% of AT bases. DNA content and ploidy level were positively correlated within this complex. In I. batatas and the more closely related species I. trifida, genome size and ploidy levels were correlated with pollen size. Our results allow us proposing alternative or complementary hypotheses to the one currently proposed for the formation of hexaploid Ipomoea batatas. This article is protected by copyright. All rights reserved.

Artificial selection experiments are designed to investigate phenotypic evolution of complex traits and its genetic basis. Here we focused on flowering time, a trait of key importance for plant adaptation and life-cycle shifts. We undertook divergent selection experiments from two maize inbred lines. After 13 generations of selection, we obtained a time-lag of roughly two weeks between Early- and Late-populations. We used this material to characterize the genome-wide transcriptomic response to selection in the shoot apical meristem before, during and after floral transition in field conditions during two consecutive years. We validated the reliability of performing RNA-sequencing in uncontrolled conditions. We found that roughly half of maize genes were expressed in the shoot apical meristem, 59.3% of which were differentially expressed. We detected a majority of genes with differential expression between inbreds and across meristem status, and retrieved a subset of 2,451 genes involved in the response to selection. Among these, we found a significant enrichment for genes with known function in maize flowering time. Furthermore, they were more often shared between inbreds than expected by chance, suggesting convergence of gene expression. We discuss new insights into the expression pattern of key players of the underlying gene regulatory network including the Zea mays genes CENTRORADIALIS (ZCN8), RELATED TO AP2.7 (RAP2.7), MADS4 (ZMM4), KNOTTED1 (KN1), GIBBERELLIN2-OXIDASE1 (GA2ox1), as well as alternative scenarios for genetic convergence.

Self-incompatibility (SI) is a genetic system found in some hermaphrodite plants. Recognition of pollen by pistils expressing cognate specificities at two linked genes leads to rejection of self pollen and pollen from close relatives, i.e., to avoidance of self-fertilization and inbred matings, and thus increased outcrossing. These genes generally have many alleles, yet the conditions allowing the evolution of new alleles remain mysterious. Evolutionary changes are clearly necessary in both genes, since any mutation affecting only one of them would result in a nonfunctional self-compatible haplotype. Here, we study diversification at the S-locus (i.e., a stable increase in the total number of SI haplotypes in the population, through the incorporation of new SI haplotypes), both deterministically (by investigating analytically the fate of mutations in an infinite population) and by simulations of finite populations. We show that the conditions allowing diversification are far less stringent in finite populations with recurrent mutations of the pollen and pistil genes, suggesting that diversification is possible in a panmictic population. We find that new SI haplotypes emerge fastest in populations with few SI haplotypes, and we discuss some implications for empirical data on S-alleles. However, allele numbers in our simulations never reach values as high as observed in plants whose SI systems have been studied, and we suggest extensions of our models that may reconcile the theory and data.

International audience; Base composition varies among and within eukaryote genomes. Although mutational bias and selection have initially been invoked, more recently GC-biased gene conversion (gBGC) has been proposed to play a central role in shaping nucleotide landscapes, especially in yeast, mammals, and birds. gBGC is a kind of meiotic drive in favor of G and C alleles, associated with recombination. Previous studies have also suggested that gBGC could be at work in grass genomes. However, these studies were carried on third codon positions that can undergo selection on codon usage. As most preferred codons end in G or C in grasses, gBGC and selection can be confounded. Here we investigated further the forces that might drive GC content evolution in the rice genus using both coding and noncoding sequences. We found that recombination rates correlate positively with equilibrium GC content and that selfing species (Oryza sativa and O. glaberrima) have significantly lower equilibrium GC content compared with more outcrossing species. As recombination is less efficient in selfing species, these results suggest that recombination drives GC content. We also detected a positive relationship between expression levels and GC content in third codon positions, suggesting that selection favors codons ending with G or C bases. However, the correlation between GC content and recombination cannot be explained by selection on codon usage alone as it was also observed in noncoding positions. Finally, analyses of polymorphism data ruled out the hypothesis that genomic variation in GC content is due to mutational processes. Our results suggest that both gBGC and selection on codon usage affect GC content in the Oryza genus and likely in other grass species.

A study of microsporogenesis (the earliest stage of pollen ontogeny) was undertaken in seven cultivars of Codiaeum variegatum var. pictum, a eudicot species that produces inaperturate pollen grains. Microsporogenesis appears highly variable for the developmental events suspected to be implicated in the determination of aperture pattern. Most eudicots have tri-aperturate pollen grains and microsporogenesis is described as highly conserved in this clade. The observed burst of variation in C. variegatum therefore appears especially remarkable. A plausible hypothesis to explain the variation is that the pollen being inaperturate, the selective forces applying on the ontogeny of the aperture pattern are relaxed.

Pollen grains display a wide range of variation in aperture number and arrangement (pattern) in angiosperms. Apertures are well-defined areas of the pollen wall surface that permit pollen tube germination. For low aperture numbers, aperture patterns are characteristic of the major taxonomic divisions of angiosperms. This paper presents a developmental model that explains most of the aperture patterns that are recorded in angiosperms. It is based on the analysis of the different events that occur during meiosis and lead to microspore differentiation. It demonstrates that variation occurring during meiosis in angiosperms is sufficient to produce the core morphological set of the most commonly observed pollen morphologies.

Background Long term selection experiments bring unique insights on the genetic architecture of quantitative traits and their evolvability. Indeed, they are utilized to (i) monitor changes in allele frequencies and assess the effects of genomic regions involved traits determinism; (ii) evaluate the role of standing variation versus new mutations during adaptation; (iii) investigate the contribution of non allelic interactions. Here we describe genetic and phenotypic evolution of two independent Divergent Selection Experiments (DSEs) for flowering time conducted during 16 years from two early maize inbred lines. Results Our experimental design uses selfing as the mating system and small population sizes, so that two independent families evolved within each population, Late and Early. Observed patterns are strikingly similar between the two DSEs. We observed a significant response to selection in both directions during the first 7 generations of selection. Within Early families, the response is linear through 16 generations, consistent with the maintenance of genetic variance. Within Late families and despite maintenance of significant genetic variation across 17 generations, the response to selection reached a plateau after 7 generations. This plateau is likely caused by physiological limits. Residual heterozygosity in the initial inbreds can partly explain the observed responses as evidenced by 42 markers derived from both Methyl-Sensitive Amplification- and Amplified Fragment Length- Polymorphisms. Among the 42, a subset of 13 markers most of which are in high linkage disequilibrium, display a strong association with flowering time variation. Their fast fixation throughout DSEs’ pedigrees results in strong genetic differentiation between populations and families. Conclusions Our results reveal a paradox between the sustainability of the response to selection and the associated dearth of polymorphisms. Among other hypotheses, we discuss the maintenance of heritable variation by few mutations with strong epistatic interactions whose effects are modified by continuous changes of the genetic background through time. Electronic supplementary material The online version of this article (doi:10.1186/s12862-015-0382-5) contains supplementary material, which is available to authorized users.

Plant genomes are large, intron-rich and present a wide range of variation in coding region G+C content. Concerning coding regions, a sort of syndrome can be described in plants: the increase in G+C content is associated with both the increase in heterogeneity among genes within a genome and the increase in variation across genes. Taking advantage of the large number of genes composing plant genomes and the wide range of variation in gene intron number, we performed a comprehensive survey of the patterns of variation in G+C content at different scales from the nucleotide level to the genome scale in two species Arabidopsis thaliana and Oryza sativa, comparing the patterns in genes with different intron numbers. In both species, we observed a pervasive effect of gene intron number and location along genes on G+C content, codon and amino acid frequencies suggesting that in both species, introns have a barrier effect structuring G+C content along genes. In external gene regions (located upstream first or downstream last intron), species-specific factors are shaping G+C content while in internal gene regions (surrounded by introns), G+C content is constrained to remain within a range common to both species. In rice, introns appear as a major determinant of gene G+C content while in A. thaliana introns have a weaker but significant effect. The structuring effect of introns in both species is susceptible to explain the G+C content syndrome observed in plants.

In a previous study, we identified a candidate fragment length polymorphism associated with flowering time variation after seven generations of selection for flowering time, starting from the maize inbred line F252. Here, we characterized the candidate region and identified underlying polymorphisms. Then, we combined QTL mapping, association mapping, and developmental characterization to dissect the genetic mechanisms responsible for the phenotypic variation. The candidate region contained the Eukaryotic Initiation Factor (eIF-4A) and revealed a high level of sequence and structural variation beyond the 3′-UTR of eIF-4A, including several insertions of truncated transposable elements. Using a biallelic single-nucleotide polymorphism (SNP) (C/T) in the candidate region, we confirmed its association with flowering time variation in a panel of 317 maize inbred lines. However, while the T allele was correlated with late flowering time within the F252 genetic background, it was correlated with early flowering time in the association panel with pervasive interactions between allelic variation and the genetic background, pointing to underlying epistasis. We also detected pleiotropic effects of the candidate polymorphism on various traits including flowering time, plant height, and leaf number. Finally, we were able to break down the correlation between flowering time and leaf number in the progeny of a heterozygote (C/T) within the F252 background consistent with causal loci in linkage disequilibrium. We therefore propose that both a cluster of tightly linked genes and epistasis contribute to the phenotypic variation for flowering time.

International audience; In flowering plants, the haploid phase is reduced to the pollen grain and embryo sac. These reproductive tissues (gametophytes) are actually distinct individuals that have a different genome from the plant (sporophyte), and are more or less independent. The morphology of pollen grains, particularly the openings permitting pollen tube germination (apertures), is crucial for determining the outcome of pollen competition. Many species of flowering plants simultaneously produce pollen grains with different aperture numbers in a single individual (heteromorphism). In this paper, we show that the heteromorphic pollen aperture pattern depends on the genetic control of pollen morphogenesis. This points out a conflict of interest between genes expressed in the sporophyte and genes expressed in the gametophyte. More generally, such a conflict should exist whenever heteromorphism is an ESS resulting from a bet-hedging strategy. For pollen aperture, heteromorphism has been observed in about 40% of angiosperm species, suggesting that conflicting situations are the rule. In this context, the sporo-gametophytic conflict could be one of the factors that led to the reduction of the haploid phase in plants. (c) 2012 Elsevier Inc. All rights reserved.

International audience; Background and Aims The tam (tardy asynchronous meiosis) mutant of Arabidopsis thaliana, which exhibits a modified cytokinesis with a switch from simultaneous to successive cytokinesis, was used to perform a direct test of the implication of cytokinesis in aperture-pattern ontogeny of angiosperm pollen grains. The aperture pattern corresponds to the number and arrangement of apertures (areas of the pollen wall permitting pollen tube germination) on the surface of the pollen grain. Methods A comparative analysis of meiosis and aperture distribution was performed in two mutant strains of arabidopsis: quartet and quartet-tam. Key Results While the number of apertures is not affected in the quartet-tam mutant, the arrangement of the three apertures is modified compared with the quartet, resulting in a different aperture pattern. Conclusions These results directly demonstrate the relationship between the type of sporocytic cytokinesis and pollen aperture-pattern ontogeny.

International audience; Premise of the study: Pollen grains of fl owering plants display a fascinating diversity of forms, in spite of their minute size. The observed diversity is determined by the developmental mechanisms implicated in the establishment of pollen morphological features. Pollen grains are generally surrounded by an extremely resistant wall interrupted in places by apertures that play a key role in reproduction, being the places at which pollen tube growth is initiated. Aperture shape, number, and position are determined during microsporogenesis (male meiosis), the earliest step in pollen ontogeny. We investigate in detail the unfolding of microsporogenesis in three species that present uncommon aperture pattern (i.e., disulculate in Calycanthus fl oridus [Calycanthaceae, magnoliids], tetraporate in Hohenbergia stellata [Bromeliaceae, monocots], and monoporate in Typha latifolia [Typhaceae, monocots]). Methods: We performed a comparative analysis of microsporogenesis and aperture distribution within tetrads in these species with contrasting aperture arrangements. This was done using aniline blue coloration and UV light microscope observations. Keys results: We show that aperture localization and features of callose deposition on intersporal walls produced during cytokinesis coincide in all three species examined. Such a correlation suggests that patterns of callose deposition are strongly involved in determining aperture localization. Conclusion: In fl owering plants, patterns of male meiosis and especially callose deposition following meiosis may be implicated in the diversity of pollen aperture patterns.

† Background and Aims In flowering plants, microsporogenesis is accompanied by various types of cytoplasmic partitioning (cytokinesis). Patterns of male cytokinesis are suspected to play a role in the diversity of aperture patterns found in pollen grains of angiosperms. The relationships between intersporal wall formation, tetrad shape and pollen aperture pattern ontogeny are studied. † Methods A comparative analysis of meiosis and aperture distribution was performed within tetrads in two triporate eudicot species with contrasting aperture arrangements within their tetrads [Epilobium roseum (Onagraceae) and Paranomus reflexus (Proteaceae)]. † Key Results and Conclusions Intersporal wall formation is a two-step process in both species. Cytokinesis is first achieved by the formation of naked centripetal cell plates. These naked cell plates are then covered by additional thick, localized callose deposits that differ in location between the two species. Apertures are finally formed in areas in which additional callose is deposited on the cell plates. The recorded variation in tetrad shape is correlated with variations in aperture pattern, demonstrating the role of cell partitioning in aperture pattern ontogeny.

International audience; This paper presents the first broad overview of three main features of microsporogenesis (male meiosis) in angiosperms: cytokinesis (cell division), intersporal wall formation, and tetrad form. A phylogenetic comparative approach was used to test for correlated evolution among these characters and to make hypotheses about evolutionary trends in microsporogenesis. The link between features of microsporogenesis and pollen aperture type was examined. We show that the pathway associated with successive cytokinesis (cytoplasm is partitioned after each meiotic division) is restricted to wall formation mediated by centrifugally developing cell plates, and tetragonal (or decussate, T-shaped, linear) tetrads. Conversely, much more flexibility is observed when cytokinesis is simultaneous (two meiotic divisions completed before cytoplasmic partitioning). We suggest that the ancestral type of microsporogenesis for angiosperms, and perhaps for all seed plants, associated simultaneous cytokinesis with centripetal wall formation, resulting in a large diversity in tetrad forms, ranging from regular tetrahedral to tetragonal tetrads, including rhomboidal tetrads. From this ancestral pathway, switches toward successive cytokinesis occurred among basal angiosperms and monocots, generally associated with a switch toward centrifugal intersporal wall formation, whereas eudicots evolved toward an almost exclusive production of regular tetrahedral tetrads. No straightforward link is found between the type of microsporogenesis and pollen aperture type.

Although the pollen grains produced in monocots are predominantly monosulcate (or monoporate), other aperture types are also found within this taxonomic group, such as the trichotomosulcate, inaperturate, zonaperturate, di-, or triaperturate types. The aperture pattern is determined during the young-tetrad stage of pollen development and it is known that some features of microsporogenesis can constrain the aperture type. For example, trichotomosulcate pollen is always associated with simultaneous cytokinesis, a condition considered as derived in the monocots. Our observations of the microsporogenesis pathway in a range of monocot species show that this pathway is surprisingly variable. Our results, however preliminary, reveal that variation in microsporogenesis concerns not only cytokinesis but also callose deposition among the microspores and shape of the tetrads. The role played by these features in aperture pattern determination is discussed.

The aperture pattern of pollen grains is a character defined as the number, shape, and position of apertures. Although this character is highly variable in angiosperms, two states are particularly widespread. Pollen grains with one polar aperture occur frequently in basal angiosperms and monocots while tricolpate pollen is a synapomorphy of the eudicots. Many morphological characters are the result of a compromise between selective forces (acting on morphology) and developmental constraints (limiting the range of possible morphologies). To investigate what are the respective roles of development and selection in the determination of aperture pattern in angiosperms, we have chosen to study the characteristics of cell division during male meiosis, since it has been shown that aperture pattern is determined during microsporogenesis. The present study focuses on Asparagales. From a selection of species belonging to the major families of Asparagales, we described the type of cytokinesis, the way callose is deposited, the shape of the tetrad, as well as the shape and position of apertures within the tetrad. We show that although pollen morphology is quite uniform in Asparagales (most species produce monosulcate pollen), the characteristics of cell division during male meiosis vary among families. A highly conserved developmental sequence is observed in higher Asparagales whereas lower Asparagales, and particularly Iridaceae, display different ways of achieving cell division.

Pollen aperture patterns vary widely in angiosperms. An increasing number of studies indicate that aperture pattern ontogeny is correlated with the way in which cytokinesis that follows male meiosis is completed. The formation of the intersporal callose walls that isolate the microspores after meiosis was studied in four species with different aperture patterns (two monocots, Phormium tenax and Asphodelus albus, and two eudicots, Helleborus foetidus and Protea lepidocarpodendron). The way in which post-meiotic cytokinesis is performed differs between all four species, and variation in callose deposition appears to be linked to aperture pattern definition.

• Background and Aims Early developmental events in microsporogenesis are known to play a role in pollen morphology: variation in cytokinesis type, cell wall formation, tetrad shape and aperture polarity are responsible for pollen aperture patterning. Despite the existence of other morphologies, monosulcate pollen is one of the most common aperture types in monocots, and is also considered as the ancestral condition in this group. It is known to occur from either a successive or a simultaneous cytokinesis. In the present study, the developmental sequence of microsporogenesis is investigated in several species of Asparagales that produce such monosulcate pollen, representing most families of this important monocot clade. • Methods The developmental pathway of microsporogenesis was investigated using light transmission and epifluorescence microscopy for all species studied. Confocal microscopy was used to confirm centripetal cell plate formation. • Key Results Microsporogenesis is diverse in Asparagales, and most variation is generally found between families. It is confirmed that the whole higher Asparagales clade has a very conserved microsporogenesis, with a successive cytokinesis and centrifugal cell plate formation. Centripetal cell wall formation is described in Tecophilaeaceae and Iridaceae, a feature that had so far only been reported for eudicots. • Conclusions Monosulcate pollen can be obtained from several developmental pathways, leading thus to homoplasy in the monosulcate character state. Monosulcate pollen should not therefore be considered as the ancestral state unless it is produced through the ancestral developmental pathway. The question about the ancestral developmental pathway leading to monosulcy remains open.

International audience; In most flowering plants, a single cytokinesis follows the two meiotic divisions during pollen-grain ontogeny. Aperture pattern (i.e., aperture number and distribution on pollen surface) ontogeny could be linked to the processes ensuring the apportionment of the cytoplasm to the four microspores. This apportionment is achieved by radial arrays of microtubules organized around the nuclei. The cleavage planes are defined in the overlapping regions of opposing arrays extending from different nuclei. We followed the establishment of these arrays in two different lines of plants belonging to the genus Nicotiana that produce pollen grains with different aperture numbers. Different distributions of the microtubules have been observed, which can be interpreted as resulting from variation in the interactions between nuclei; these distributions appear to be correlated with aperture number. As a consequence, we propose that simultaneous cytokinesis allows the formation of multiple pollen morphologies. This mechanism is consistent with aperture number distribution within angiosperms and provides clues to help our understanding of the evolution of aperture number.

International audience; Pontederia cordata L. and Eichhornia paniculata (Spreng.) Solms (Pontederiaceae, monocots) produce disulculate pollen grains. The formation of intersporal callose walls was followed in these two species by staining callose during male meiosis. A new type of cytokinesis that ends with the formation of conspicuous callose knobs at future aperture sites was observed. This observation indicates that the formation of the disulculate pattern is linked to cytokinesis. Thus, equatorial aperture patterns of monocots and aperture patterns of eudicots appear to be defined by similar mechanisms.

BackgroundIn order to investigate the rate and limits of the response to selection from highly inbred genetic material and evaluate the respective contribution of standing variation and new mutations, we conducted a divergent selection experiment from maize inbred lines in open-field conditions during 7 years. Two maize commercial seed lots considered as inbred lines,F252 andMBS847, constituted two biological replicates of the experiment. In each replicate, we derived an Early and a Late population by selecting and selfing the earliest and the latest individuals, respectively, to produce the next generation.ResultsAll populations, except the EarlyMBS847, responded to selection despite a short number of generations and a small effective population size. Part of the response can be attributed to standing genetic variation in the initial seed lot. Indeed, we identified one polymorphism initially segregating in theF252 seed lot at a candidate locus for flowering time, which explained 35% of the trait variation within the LateF252 population. However, the model that best explained our data takes into account both residual polymorphism in the initial seed lots and a constant input of heritable genetic variation by new (epi)mutations. Under this model, values of mutational heritability range from 0.013 to 0.025, and stand as an upper bound compare to what is reported in other species.ConclusionsOur study reports a long-term divergent selection experiment for a complex trait, flowering time, conducted on maize in open-field conditions. Starting from a highly inbred material, we created within a few generations populations that strikingly differ from the initial seed lot for flowering time while preserving most of the phenotypic characteristics of the initial inbred. Such material is unique for studying the dynamics of the response to selection and its determinants. In addition to the fixation of a standing beneficial mutation associated with a large phenotypic effect, a constant input of genetic variance by new mutations has likely contributed to the response. We discuss our results in the context of the evolution and mutational dynamics of populations characterized by a small effective population size.