A genome-wide survey of reproductive barriers in an intraspecific hybrid

Genetic study of the reproductive barriers between related species plays an essential role in understanding the process of speciation. We developed a new method for mapping all possible factors causing deviations from expected Mendelian segregation ratios in F(2) progeny, which substantially contribute to reproductive isolation. A multiresponse nonlinear regression analysis of the allele frequencies of the markers covering an entire genome in the F(2) population was performed to estimate the map position and intensity of the reproductive barriers on each chromosome. In F(2) plants from a cross between a Japonica variety of rice, Nipponbare, and an Indica variety, Kasalath, the deviations of allele frequencies were well explained by 33 reproductive barriers. Of these, 15 reproductive barriers affected the allele transmission rate through the gametophyte and in 9 of these 15 cases, an Indica allele was transmitted at a higher frequency than a Japonica allele. The other 18 reproductive barriers altered the viability of the zygote via its genotype. Two zygotic reproductive barriers showed overdominance and 5 showed underdominance. The most pronounced reproductive barrier, mapped at 62.3 +/- 0.4 cM on chromosome 3, transmitted the Indica allele by 94% through the male gametophyte. The accuracy of the barrier position in the regression analysis was confirmed by progeny analysis. The regression analysis proved to be a powerful tool for detecting and characterizing every reproductive barrier, irrespective of whether it acted on the male or female gametophyte or the zygote.     

Diverse reproductive barriers in hybridising crickets suggests extensive variation in the evolution and maintenance of isolation

Reproductive barriers reduce gene flow between populations and maintain species identities. A diversity of barriers exist, acting before, during and after mating. To understand speciation and coexistence, these barriers need to be quantified and their potential interactions revealed. We use the hybridising field crickets Gryllus bimaculatus and G. campestris as a model to understand the full compliment and relative strength of reproductive barriers. We find that males of both species prefer conspecific females, but the effect is probably too weak to represent a barrier. In contrast, prezygotic barriers caused by females being more attracted to conspecific male song and preferentially mounting and mating with conspecifics are strong and asymmetric. Postzygotic barriers vary in direction; reductions in fecundity and egg viability create selection against hybridisation, but hybrids live longer than pure-bred individuals. Hybrid females show a strong preference for G. bimaculatus songs, which together with a complete lack of hybridisation by G. campestris females, suggests that asymmetric gene flow is likely. For comparison, we review reproductive barriers that have been identified between other Gryllids and conclude that multiple barriers are common. Different species pairs are separated by qualitatively different combinations of barriers, suggesting that reproductive isolation and even the process of speciation itself may vary widely even within closely related groups.     

Relationship between transmission ratio distortion and genetic divergence in intraspecific rice crosses

The strength of reproductive isolation often correlates positively with parental divergence in both animals and plants. Here, we assess the relationship between transmission ratio distortion (TRD) of marker loci and parental divergence in intraspecific rice (Oryza sativa L.) crosses. We produced 10 diverse F(2) populations by crossing a temperate japonica reference accession with each of 10 donor accessions belonging to subpopulations different from the reference accession, and then genotyped the F(2) populations using molecular markers distributed across the entire genome. Significant TRDs (α?=?0.05) were detected in 9 of the 10 F(2) populations. TRD regions on chromosomes 3 and 6 were common to several populations; in contrast, other TRD regions were unique to a single population, indicating the diversification of genomic location of TRDs among the populations. The level of TRD (estimated from the overall number and magnitude of TRDs) was significantly correlated with the genetic distance between the donor accessions and the reference accession. Our results suggest that in intraspecific rice crosses, parental divergence may result in diversification of the TRD pattern, followed by an increase in the level of TRD.     

A resource budget model to explain intraspecific variation in mast reproductive dynamics

Rowan, Sorbus aucuparia L., exhibits conspicuous geographic variation in its inter-annual patterns of reproduction across southern Norway. Along the southern west coast, trees flower and produce clusters of succulent berries every other year, while in the east, trees mast every 3 years. We investigate two hypotheses for this variation: local adaptation to environmental conditions that favors different reproductive schedules at different locations or variation arising from single stereotyped reproductive strategy in the face of a geographic variation in productivity. To assess the theoretical plausibility of each of these hypotheses, we develop a refined resource budget model for mast reproduction. The refined model assumes that there is a lower bound on the amount of resource that may be allocated to reproduction, which allows the prediction of the relation between reproductive schedules and productivity. From the analysis of the model, we find that the observed geographic transition in rowan mast is caused if either productivity in the east is lower than that in the west or if plants in the eastern populations have evolved to pause reproduction until energy reserves are replenished to higher levels. The prediction by the productivity gradient hypothesis match with the empirical finding that the productivity is, in fact, likely to be lower in the east than the west, although we lack empirical data to test the likelihood of the local adaptation hypothesis. There is a need for experimental studies to clarify the validity of the local adaptation hypothesis. Akiko Satake is the recipient of the 11th Denzaburo Miyadi Award.     

Effects of intraspecific competition on size variation and reproductive allocation in a clonal plant

Clonal plants grow in diameter rather than height, and therefore competition among genets is likely to be symmetric and to result in smaller variation in size of genets than in non-clonal plants. Moreover, clonal plants can reproduce both sexually and vegetatively. We studied the effects of density on the size of rosettes and of clones, variation in the size of rosettes and of clones, and allocation to sexual and vegetative reproduction in the clonal herb Ranunculus reptans . We grew plants from an artificial population of R. reptans in 32 trays at two densities. After four months, differences in density were still apparent, although clones in the low-density treatment had on average 155% more rosettes and 227% more rooted rosettes than clones in the high-density treatment. The coefficient of variation of these measures of clone size was 15% and 83% higher, respectively, in the low-density treatment. This indicates that intraspecific competition among clones of R. reptans is symmetric and increases the effective population size. Rooted rosettes were larger and varied more in size in the low-density treatment. The relative allocation of the populations to sexual and to vegetative reproduction was 19% and 13% higher, respectively, in the high-density treatment. Moreover, seeds produced in the high-density treatment had a 24% higher mass and a 7% higher germination percentage. This suggests that with increasing density, allocation to sexual reproduction increases more than allocation to vegetative reproduction in R. reptans , which corresponds to the response of some other species with a spreading growth form but not of species with a compact growth form. We conclude that intraspecific competition is an important factor in the life-history evolution of R. reptans because intraspecific competition affects its clonal life-history traits and may affect evolutionary processes such as genetic drift and selection through its effect on the effective population size.     

Comparison of single seed descent and anther culture-derived lines of three single crosses of rice

Summary Distribution parameters (mean, variance, skewness and kurtosis) of 12 quantitative traits were evaluated for inbred lines generated through single seed descent (SSD) and anther culture (AC) of two japonica x japonica hybrids and one japonica x indica hybrid of rice. For most of the traits the data were normally distributed, and the means and variances were found to be identical for SSD- and AC-derived lines of the given hybrids. However, for some other traits, differences between the two population types were observed, mainly in the lines derived from the intra-japonica crosses. A tentative explanation for these differences is given. Nevertheless, our results suggest that SSD and AC are equally effective breeding techniques for producing agronomically useful lines of rice.     

Inter- and intraspecific variation in the life histories of three sympatric isopods in a Hong Kong forest

There is a paucity of data on the life-history strategies of tropical animals. Accordingly, the inter and intraspecific life-history variations of three sympatric isopods, Burmoniscus oceliutus (Philosciidae), Formosillo raffaelei and Orodillo maculatus (Armadillidae), were studied at two sites in a Hong Kong forest. All three isopods were iteroparous. Burmoniscus oceilutus had more frequent breeding bouts, and smaller broods than O. muculutus and F. raffaelei. Although the three isopods had different life spans, brood numbers and life-time fecundities, their rates of offspring production (number of eggs per month) were similar. The Hong Kong isopods had longer breeding periods, a greater number of broods per life time, and lower reproductive allocation per brood than their temperate counterparts. These findings were consistent with the predictions of r - and K-theory. Intraspecific, between-site differences in fecundity and brood dry weight of F. raffaelei could probably be explained by the variations in the size of gravid females. Burmoniscus ocellutus produced heavier eggs at the site with a higher population density, suggesting that conditions at the site with a higher isopod density may be more K-selecting. Female-biased sex ratios, possibly resulting from differential sex-specific mortality, were common among the isopod samples, and the possible adaptive significance of this pattern was discussed. The potential influence of phylogenetic constraints on the evolution of life-history strategies was also considered.     

Prezygotic barriers to hybridization in marine broadcast spawners: reproductive timing and mating system variation

Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides). Interspecific patterns of egg release were coincident at seasonal (single peak in spring to early summer) to semilunar timescales. Synthesis of available data indicated that spawning is controlled by semidiurnal tidal and daily light-dark cues, and not directly by semilunar cycles. Importantly, interspecific shifts in timing detected at the hourly scale during single tides were consistent with a partial ecological prezygotic hybridization barrier. The species displayed patterns of gamete release consistent with a power law distribution, indicating a high degree of reproductive synchrony, while the hypothesis of weaker selective constraints for synchrony in selfing versus outcrossing species was supported by observed spawning in hermaphrodites over a broader range of tidal phase than in outcrossers. Synchronous gamete release is critical to the success of external fertilization, while high-energy intertidal environments may offer only limited windows of reproductive opportunity. Within these windows, however, subtle variations in reproductive timing have evolved with the potential to form ecological barriers to hybridization.     

Ecological consequences of intraspecific variation in lake Daphnia

1. Although populations harbour considerable diversity, most ecological studies still assume they are homogeneous. However, mounting evidence suggests that intraspecific diversity is not only common, but also important for interactions with community members. Here, intraspecific variation in Daphnia dentifera in haemoglobin content is shown to be a marker of hypolimnion use. 2. Hypolimnion use differed substantially within and among D. dentifera populations. Daphnia dentifera with haemoglobin resided primarily in the hypolimnion, while D. dentifera lacking haemoglobin migrated vertically. These ‘deep’ and ‘migratory’D. dentifera had different seasonal phenologies and dynamics. 3. Deep and migratory D. dentifera had qualitatively different relationships with an important competitor, Daphnia pulicaria. Deep D. dentifera density was negatively correlated with D. pulicaria density, whereas migratory density was not correlated with D. pulicaria density. 4. Given that D. pulicaria tends to reside in the hypolimnion, this negative correlation probably reflects competition between D. pulicaria and the deep D. dentifera. This pattern would have been missed if only the relationship between the overall lake populations of D. dentifera and D. pulicaria had been studied. 5. Abundances of deep D. dentifera and D. pulicaria were both correlated with the size of the hypolimnetic refuge from fish predation, but in opposite directions. Lakes with large refuges generally had high D. pulicaria and low deep D. dentifera densities.     

Plant age affects intraspecific variation in functional traits

Functional traits are often used to examine ecological patterns and processes. Ontogeny—changes that occur over time as the result of development—generates variation in traits within individual organisms. We aimed to quantify the role of ontogeny in structuring functional trait variation across a range of co-existing herbaceous perennial species and hypothesized that ontogenetic variation in traits would be greater in younger vs. older plants. We grew eight herbaceous perennial forb species common in tallgrass prairies from seed in a greenhouse in Madison, Wisconsin, USA to determine how and when time-related variation in functional traits is large relative to other sources of variation, such as differences between leaves and species. We destructively measured common functional traits on four individuals of each species every two weeks for 19 weeks, including leaf mass fraction, root mass fraction, stem mass fraction, specific leaf area, leaf dry matter content, and leaf area. We found that most functional traits indeed change through time, that the direction of many changes are consistent between species but the magnitude of change is species specific, and most time-related variation occurred earlier in development. These results emphasize the importance of considering sampling timing and differences between young and old plants when measuring functional traits. Our results suggest that ontogenetic intraspecific variation can be substantial, especially early in life. It may be problematic to use traits measured from mature plants to interpret the importance of processes that occur at earlier life stages or vice versa; using seedling traits to understand adult plant responses may also be inappropriate.

     

Lack of intraspecific variation in cpDNA in Trichocolea tomentella

Abstract

The Asian genus Struckia Müll.Hal. is reduced to a synonym of Plagiothecium Bruch &; Schimp. according to a phylogenetic analysis involving S. argentata (Mitt.) Müll.Hal., S. enervis (Broth.) Ignatov, T.J.Kop. &; D.G.Long and 13 representative boreal species of Plagiothecium. Two nuclear regions, ITS and partial gapC, and five chloroplast regions, rbcL, rps4-trnS, psaB, trnG and trnL-F, were utilized to reconstruct the phylogenetic relationship between Struckia and Plagiothecium. Bayesian, maximum parsimony, and maximum likelihood analyses resulted in a strongly supported clade including Struckia and Plagiothecium. Plagiothecium handelii Broth. and P. paleaceum (Mitt.) A.Jaeger, which share similar geographical ranges with Struckia and intermediate morphological traits between Struckia and Plagiothecium, grouped with the Chinese Struckia, and all of them appeared as sister to the ‘core’ of Plagiothecium. The accurate position of P. piliferum (Sw.) Schimp. remains unclear due to moderate support from all analyses, and we suggest that it is retained in Plagiothecium until further evidence is available.     

Nanostructure predicts intraspecific variation in ultraviolet-blue plumage colour

Evidence suggests that structural plumage colour can be an honest signal of individual quality, but the mechanisms responsible for the variation in expression of structural coloration within a species have not been identified. We used full-spectrum spectrometry and transmission electron microscopy to investigate the effect of variation in the nanostructure of the spongy layer on expression of structural ultraviolet (UV)-blue coloration in eastern bluebird (Sialia sialis) feathers. Fourier analysis revealed that feather nanostructure was highly organized but did not accurately predict variation in hue. Within the spongy layer of feather barbs, the number of circular keratin rods significantly predicted UV-violet chroma, whereas the standard error of the diameter of these rods significantly predicted spectral saturation. These observations show that the precision of nanostructural arrangement determines some colour variation in feathers.     

Why intraspecific trait variation matters in community ecology

Natural populations consist of phenotypically diverse individuals that exhibit variation in their demographic parameters and intra- and inter-specific interactions. Recent experimental work indicates that such variation can have significant ecological effects. However, ecological models typically disregard this variation and focus instead on trait means and total population density. Under what situations is this simplification appropriate? Why might intraspecific variation alter ecological dynamics? In this review we synthesize recent theory and identify six general mechanisms by which trait variation changes the outcome of ecological interactions. These mechanisms include several direct effects of trait variation per se and indirect effects arising from the role of genetic variation in trait evolution.     

Sex-independent transmission ratio distortion system responsible for reproductive barriers between Asian and African rice species

* A sex-independent transmission ratio distortion (siTRD) system detected in the interspecific cross in rice was analyzed in order to understand its significance in reproductive barriers. The S(1) gene, derived from African rice Oryza glaberrima, induced preferential abortion of both male and female gametes possessing its allelic alternative (), from Asian rice O. sativa, only in the heterozygote. * The siTRD was characterized by resolving it into mTRD and fTRD occurring through male and female gametes, respectively, cytological analysis of gametophyte development, and mapping of the S(1) locus using near-isogenic lines. The allelic distribution of the S(1) locus in Asian and African rice species complexes was also analyzed. * The siTRD system involved at least two components affecting male and female gametogeneses, respectively, including a modifier(s) that enhances fTRD. The chromosomal location of the major component causing the mTRD was delimited within an approx. 40 kb region. The S(1) locus induced hybrid sterility in any pairwise combination between Asian and African rice species complexes. * The allelic state of the S(1) locus has diverged between Asian and African rice species complexes, suggesting that the TRD system has a significant role in the reproductive barriers in rice.     

Inter and intraspecific variation on reproductive phenology of the Brazilian Atlantic forest Rubiaceae: ecology and phylogenetic constraints

The reproductive phenology of seven species of Rubiaceae from the Brazilian Atlantic rain forest was compared to evaluate the occurrence of phylogenetic constraints on flowering and fruiting phenologies. Since phenological patterns can be affected by phylogenetic constraints, we expected that reproductive phenology would be similar among plants within a family or genus, occurring during the same time (or season) of the year. Observations on flowering and fruiting phenology were carried out monthly, from December 1996 to January 1998, at Núcleo Picinguaba, Parque Estadual da Serra do Mar, Ubatuba, S?o Paulo State, Brazil. Nine phenological variables were calculated to characterize, quantify and compare the reproductive phenology of the Rubiaceae species. The flowering patterns were different among the seven species studied, and the Kruskal-Wallis test indicated significant differences in flowering duration first flowering, peak flowering and flowering synchrony. The peaks and patterns of fruiting intensity were different among the Rubiaceae species studied and they differed significantly from conspecifics in the phenological variables fruiting duration, fruiting peak date, and fruiting synchrony (Kruskal-Wallis test). Therefore, we found no evidence supporting the phylogenetic hypotheses, and climate does not seem to constrain flowering and fruiting patterns of the Rubiaceae species in the understory of the Atlantic forest.     

Incorporating intraspecific variation in tests of trait-based community assembly

Environmental filtering and niche differentiation are processes proposed to drive community assembly, generating nonrandom patterns in community trait distributions. Despite the substantial intraspecific trait variation present in plant communities, most previous studies of trait-based community assembly have used species mean trait values and therefore not accounted for intraspecific variation. Using a null model approach, I tested for environmental filtering and niche differentiation acting on three key functional traits??vegetative height, specific leaf area (SLA), and leaf dry matter content (LDMC)??in old-field plant communities. I also examined how accounting for intraspecific variation at the among-plot and individual levels affected the detection of nonrandom assembly patterns. Tests using fixed species mean trait values provided evidence of environmental filtering acting on height and SLA and niche differentiation acting on SLA. Including plot-level intraspecific variation increased the strength of these patterns, indicating an important role of intraspecific variation in community assembly. Tests using individual trait data indicated strong environmental filtering acting on all traits, but provided no evidence of niche differentiation, although these signals may have been obscured by the effects of dispersal limitation and spatial aggregation of conspecific individuals. There was also strong evidence of nonrandom assembly of individuals within single species, with the strength of environmental filtering varying among species. This study demonstrates that, while analyses using fixed species mean trait values can provide insights into community assembly processes, accounting for intraspecific variation provides a more complete view of communities and the processes driving their assembly.     

Molecularmarker diversity and hybrid sterility in indica-japonica rice crosses

 The partial sterility of hybrids between the indica and japonica rice subspecies of Asian cultivated rice is a serious constraint for utilizing inter-subspecific heterosis in hybrid rice breeding. In this study, we have investigated the relationship between molecular-marker polymorphism and indica-japonica hybrid fertility using a diallel set involving 20 rice accessions including 9 indica and 11 japonica varieties. Spikelet fertility of the resulting 190 F₁s and their parents was examined in a replicated field trial. Intra-subspecific hybrids showed much higher spikelet fertility than inter-subspecific hybrids except in crosses involving wide-compatibility varieties. The parents were surveyed for DNA polymorphism using 96 RFLP and ten SSR markers, which revealed extensive genetic differentiation between indica and japonica varieties. A large number of markers detected highly significant effects on hybrid fertility. The chromosomal locations for many of the positive markers coincided well with previously identified loci for hybrid sterility. The correlation between hybrid fertility and parental distance was low in both intra- and inter-subspecific crosses. The results suggest that the genetic basis of indica-japonica hybrid sterility is complex. It is the qualitative, rather than the quantitative, difference between the parents that determines the fertility of hybrids. Received: 3 January 1997/Accepted: 17 January 1997     

Hybridization and pre-zygotic reproductive barriers in Plasmodium

Sexual reproduction is an obligate step in the life cycle of many parasites, including the causative agents of malaria (Plasmodium). Mixed-species infections are common in nature and consequently, interactions between heterospecific gametes occur. Given the importance of managing gene flow across parasite populations, remarkably little is understood about how reproductive isolation between species is maintained. We use the rodent malaria parasites P. berghei and P. yoelii to investigate the ecology of mixed-species mating groups, identify proteins involved in pre-zygotic barriers, and examine their evolution. Specifically, we show that (i) hybridization occurs, but at low frequency; (ii) hybridization reaches high levels when female gametes lack the surface proteins P230 or P48/45, demonstrating that these proteins are key for pre-zygotic reproductive isolation; (iii) asymmetric reproductive interference occurs, where the fertility of P. berghei gametes is reduced in the presence of P. yoelii and (iv) as expected for gamete recognition proteins, strong positive selection acts on a region of P230 and P47 (P48/45 paralogue). P230 and P48/45 are leading candidates for interventions to block malaria transmission. Our results suggest that depending on the viability of hybrids, applying such interventions to populations where mixed-species infections occur could either facilitate or hinder malaria control.