Inbreeding depression in two species of Mimulus (Scrophulariaceae) with contrasting mating systems

We examined the effect of self- and cross-pollination on germination success, flowering probability, pollen and ovule production, survivorship, and adult aboveground biomass in two species of Mimulus with contrasting mating systems: the highly seifing M. micranthus and an outcrossing population of M. guttatus. Cross-pollinations were performed both within and between populations in order to examine the scale at which the genetic load is distributed. We found significant inbreeding depression in M. guttatus in four of the six traits, with the highest inbreeding depression observed in biomass (68% and 69% based on within- and between-population crosses, respectively) and lowest in ovule production (21% based on between-population crosses only). M. micranthus displayed significant inbreeding depression in only two of the six traits examined. Again, we observed the highest inbreeding depression in biomass (47–60% based on within- and between-population crosses, respectively), but both traits showing significant differences between self and outcross progeny expressed lower inbreeding depression than in M. guttatus. We detected no significant inbreeding depression for either pollen or ovule production in M. micranthus. An estimate of total inbreeding depression based on the multiplicative effects of all traits was also lower in M. micanthus than∗∗∗ in M. guttatus. Our results are consistent with the expected purging of genetic load in populations with high selfing rates. The absence of inbreeding depression in M. micranthus pollen and ovule production, two traits with strong links to fitness in a selfing annual, further suggests the important role of directional selection in determining the population's genetic load. Comparison of cross-pollinations made within and between populations revealed little evidence of divergence of genetic load among the M. micranthus and M. guttatus populations examined.     

RECONSTRUCTING ORIGINS OF LOSS OF SELF‐INCOMPATIBILITY AND SELFING IN NORTH AMERICAN ARABIDOPSIS LYRATA: A POPULATION GENETIC CONTEXT

Theoretical and empirical comparisons of molecular diversity in selfing and outcrossing plants have primarily focused on long‐term consequences of differences in mating system (between species). However, improving our understanding of the causes of mating system evolution requires ecological and genetic studies of the early stages of mating system transition. Here, we examine nuclear and chloroplast DNA sequences and microsatellite variation in a large sample of populations of Arabidopsis lyrata from the Great Lakes region of Eastern North American that show intra‐ and interpopulation variation in the degree of self‐incompatibility and realized outcrossing rates. Populations show strong geographic clustering irrespective of mating system, suggesting that selfing either evolved multiple times or has spread to multiple genetic backgrounds. Diversity is reduced in selfing populations, but not to the extent of the severe loss of variation expected if selfing evolved due to selection for reproductive assurance in connection with strong founder events. The spread of self‐compatibility in this region may have been favored as colonization bottlenecks following glaciation or migration from Europe reduced standing levels of inbreeding depression. However, our results do not suggest a single transition to selfing in this system, as has been suggested for some other species in the Brassicaceae.     

Consequences of multiple mating‐system shifts for population and range‐wide genetic structure in a coastal dune plant

Evolutionary transitions from outcrossing to selfing can strongly affect the genetic diversity and structure of species at multiple spatial scales. We investigated the genetic consequences of mating‐system shifts in the North American, Pacific coast dune endemic plant Camissoniopsis cheiranthifolia (Onagraceae) by assaying variation at 13 nuclear (n) and six chloroplast (cp) microsatellite (SSR) loci for 38 populations across the species range. As predicted from the expected reduction in effective population size (N_e) caused by selfing, small‐flowered, predominantly selfing (SF) populations had much lower nSSR diversity (but not cpSSR) than large‐flowered, predominantly outcrossing (LF) populations. The reduction in nSSR diversity was greater than expected from the effects of selfing on N_e alone, but could not be accounted for by indirect effects of selfing on population density. Although selfing should reduce gene flow, SF populations were not more genetically differentiated than LF populations. We detected five clusters of nSSR genotypes and three groups of cpSSR haplotypes across the species range consisting of parapatric groups of populations that usually (but not always) differed in mating system, suggesting that selfing may often initiate ecogeographic isolation. However, lineage‐wide genetic variation was not lower for selfing clusters, failing to support the hypothesis that selection for reproductive assurance spurred the evolution of selfing in this species. Within three populations where LF and SF plants coexist, we detected genetic differentiation among diverged floral phenotypes suggesting that reproductive isolation (probably postzygotic) may help maintain the striking mating‐system differentiation observed across the range of this species.     

Genetics of sex allocation in Mimulus (Scrophulariaceae)

Theoretical models of the evolution of resource allocation patterns to male and female function make the assumption that there are inherent trade-offs between the two. Here we use a quantitative genetic approach to quantify trade-offs between male and female function and to determine whether plant populations could readily respond to natural selection by quantifying the amount of genetic variation for pollen and ovule production. Both intra- and interspecific crossing designs were applied to two populations of the predominantly outcrossing Mimulus guttatus and two populations of the highly selfing congener, M. micranthus. The only significant correlations observed among pollen number, pollen size and ovule number were positive. Positive genetic correlations among the traits were sometimes reduced after removing the effect of flower size but still no significant negative correlations were detected. These results suggest that positive correlations between pollen and ovule production may be due to the joint positive correlation of these characters with the resource pool available for pollen and ovule production, as reflected by flower size. Heritabilities were moderate to high for ovule production but low for pollen number and pollen size and suggest that responses to selection would differ between the two traits. Crosses between the species revealed that there are additional genetic factors contributing to differences between the two species for corolla width, vs. pollen:ovule ratio. This is consistent with the hypothesis that genetic variation for resource acquisition may in part be responsible for the overall lack of a negative correlation between pollen and ovule production and provides a genetic explanation for little evidence of trade-offs between sexual functions in Mimulus.     

VARIATION OF SEX ALLOCATION AMONG EIGHT TAXA OF THE MIMULUS GUTTATUS SPECIES COMPLEX (SCROPHULARIACEAE)

Characters related to sex allocation and the mating system were studied in eight California taxa of the Mimulus guttatus complex: M. guttatus, M. nasutus, M. glaucescens, M. Tilingii, M. nudatus, M. laciniatus, M. platycalyx, and M. micranthus, ranked in approximate decreasing levels of outbreeding. Dry weights and lengths of floral parts, pollen and ovule number, and timing of stigmatic closure were measured on plants in the growth chamber. As percent of total flower weight, allocation to stamens and corollas was lowest in M. micranthus (28%), intermediate in M. platycalyx, M. Tilingii, and M. laciniatus (50%), and high in other taxa (60%). Among M. micranthus, M. platycalyx, and M. laciniatus, pollen–ovule ratios ranged from 3.9 to 12.0; ratios for other taxa were 19.3 to 26.6. Taxa with increased male biomass allocation generally show increased outcrossing and increased P/O ratios. Stigma-anther separation and closure of stigma lobes upon touch were positively correlated with outcrossing ratios, P/O ratios, and male allocation. Isozyme variation indicates the inbreeding taxa have been independently derived; thus, these associations of maleness with outbreeding are significant trends in the Mimulus guttatus complex.     

Inbreeding depression in an asexual population of Mimulus guttatus

The reproductive mechanism, that is whether an organism outcrosses, selfs or asexually reproduces, has a substantial impact on the amount and pattern of genetic variation. In this study, we estimate genetic variation and genetic load for a predominately asexual population of Mimulus guttatus, and then compare our results to other studies of predominately sexually reproducing (outcrossing and selfing) populations of M. guttatus. The asexual population had low levels of heterozygosity (H_e = 0.03) and low (but significantly non‐zero) inbreeding load, especially when compared with other M. guttatus populations. This differs greatly from the sexual populations of Mimulus that display substantial inbreeding depression. We discuss a variety of reasons why we see such low load in this study and suggest future research projects to further explore the questions.     

REPRODUCTIVE ASSURANCE MECHANISMS IN THREE TAXA OF THE MIMULUS GUTTATUS COMPLEX (SCROPHULARIACEAE)

The evolution of inbreeding in plants has often been attributed to selection for the ability to set seed in the absence of mates or pollinators. Mechanisms of reproductive assurance in five populations of mixed mating Mimulus guttatus, three populations of inbreeding M. platycalyx, and two populations of inbreeding M. nasutus were examined in a pollinator-free greenhouse. Reproductive assurance was manifested in all populations by autofertility, vegetative reproduction, or both. The inbreeding taxa had significantly greater levels of autofertility and less vegetative reproduction. Three modes of autofertility were identified: 1) due to corolla abscission only, occurring in three M. guttatus populations; 2) due to both corolla abscission and direct anther-stigma contact by curling of the lower stigmatic lobe into the anthers, occurring in two M. guttatus populations; and 3) direct stigma-anther contact by stigma curling alone prior to corolla abscission, found in each M. platycalyx and M. nasutus population. Stigma-anther distance and its interaction with stigma curling contributed to differences in autofertility among populations. Significant levels of intrapopulation quantitative genetic variation were found for seven of ten traits examined; average levels were similar between inbreeding and mixed mating populations. Genetic variation within populations for autofertility per se was not detected, but significant levels controlling stigma-anther distance were found in two M. guttatus populations. These results show that evolution of inbreeding by natural selection for reproductive assurance is possible in Mimulus, and illustrate the complex changes in floral dynamics and morphology it may involve.     

Genetic variation and the mating system in the rare Acacia sciophanes compared with its common sister species Acacia anfractuosa (Mimosaceae)

Acacia sciophanes is an extremely rare and Critically Endangered species known from two small populations separated by less than 7 km. Specifically we aimed to investigate whether rarity in A. sciophanes is associated with decreased levels of genetic variation and increased levels of selfing by comparing patterns of genetic variation and mating system parameters with its widespread and common sister species A. anfractuosa. Fourteen polymorphic allozyme loci were used to assess genetic diversity with four of these used in the estimation of mating system parameters. At the species level A. sciophanes has lower allelic richness, polymorphism, observed heterozygosity and gene diversity than A. anfractuosa and significantly lower levels of gene diversity at the population level. Both species have a mixed mating system but the largest population of A. sciophanes has lower levels of outcrossing, higher correlated paternity and increased bi-parental inbreeding compared with two A. anfractuosa populations. However, both correlated paternity and bi-parental inbreeding appear to be at least partly influenced by population size regardless of the species. We suggest that A. sciophanes is likely to be an intrinsically rare species and that in particular the lower levels of genetic diversity and increased selfing are a feature of a species that has the ability to persist in a few localised small populations. Despite recent extensive habitat destruction our comparative study provided no clear evidence that such events have contributed to the lower genetic diversity and increased selfing in A. sciophanes and we believe its ability to exist in small populations may not only be an important factor in its survival as a rare species but also indicates that it may be less susceptible to the impacts of habitat loss and fragmentation. The key to this species conservation will be the maintenance of suitable habitat, particularly through improved fire regimes and control of invasive weeds, that will allow the two small populations to continue to persist in extremely restricted areas of remnant vegetation.     

MULTIPLE PATERNITY AND SELF-FERTILIZATION IN RELATION TO FLORAL AGE IN MIMULUS GUTTATUS (SCROPHULARIACEAE)

To gain some understanding of the mechanisms responsible for the intermediate levels of both multiple paternity and selfing observed previously in the common monkeyflower, Mimulus guttatus, we performed a field experiment to assess the role of 1) multiple paternity through sequential pollinator visits and 2) selfing through corolla abscission. In M. guttatus, flowers remain open for several days; then, in the process of corolla abscission anthers drag past the stigma. We predicted that multiple visitation by pollinators over this period should increase the degree of multiple paternity of sibling arrays, and that corolla dragging may be the primary cause of the observed selfing. In three northern California populations corollas were removed after being open for 1 day (preventing further mating), and the type of matings resulting was compared to matings from adjacent flowers with undisturbed corollas. On average, seed set was twice as great in flowers with unmanipulated corollas, indicating that about half of all matings in the lifespan of a M. guttatus flower occur after 1 day. Pollen supplementation revealed that pollen was limiting in one population. Electrophoretic assay of progeny showed multiple paternity was greater in unmanipulated flowers in the two populations that were not pollen limited, suggesting that sequential pollination may be an important source of multiple paternity. However, no difference was detected in the selfing rate between the two treatments, suggesting that corolla dragging was not a source of selfing even under pollen-limited conditions.     

THE EFFECTS OF FIVE GENERATIONS OF ENFORCED SELFING ON POTENTIAL MALE AND FEMALE FUNCTION IN MIMULUS GUTTATUS

In prior work we detected no significant inbreeding depression for pollen and ovule production in the highly selfing Mimulus micranthus, but both characters showed high inbreeding depression in the mixed-mating M. guttatus. The goal of this study was to determine if the genetic load for these traits in M. guttatus could be purged in a program of enforced selfing. These characters should have been under much stronger selection in our artificial breeding program than previously reported characters such as biomass and total flower production because, for example, plants unable to produce viable pollen could not contribute to future generations. Purging of genetic load was investigated at the level of both the population and the individual maternal line within two populations of M. guttatus. Mean ovule number, pollen number, and pollen viability declined significantly as plants became more inbred. The mean performance of outcross progeny generated from crosses between pairs of maternal inbred lines always exceeded that of self progeny and was fairly constant for each trait through all five generations. The consistent performance of outcross progeny and the universally negative relationships between performance and degree of inbreeding are interpreted as evidence for the weakness of selection relative to the quick fixation of deleterious alleles due to drift during the inbreeding process. The selective removal (purging) of deleterious alleles from our population would have been revealed by an increase in performance of outcross progeny or an attenuation of the effects of increasing homozygosity. The relationships between the mean of each of these traits and the expected inbreeding coefficient were linear, but one population displayed a significant negative curvilinear relationship between the log of male fertility (a function of pollen number and viability) and the inbreeding coefficient. The generally linear form of the responses to inbreeding were taken as evidence consistent with an additive model of gene action, but the negative curvilinear relationship between male fertility and the inbreeding coefficient suggested reinforcing epistasis. Within both populations there was significant genetic variation among maternal lineages for the response to inbreeding in all traits. Although all inbred lineages declined at least somewhat in performance, several maternal lines maintained levels of performance just below outcross means even after four or five generations of selfing. We suggest that selection among maternal lines will have a greater effect than selecting within lines in lowering the genetic load of populations.     

Insights Gained From 50 Years of Studying the Evolution of Self-Compatibility in <Emphasis Type="Italic">Leavenworthia</Emphasis> (Brassicaceae)

The shift from outcrossing to selfing is one of the most common evolutionary trends in plants, and there is intense interest in why this is so. The genus Leavenworthia has been the focus of research on this question for half of a century, with particular attention paid to the evolution of self-compatibility from self-incompatibility. In this review, we discuss the last 50 years of research concerning this evolutionary transition in Leavenworthia. Selfing appears to have evolved independently at minimum three times within this genus of eight species. Work on the ecological basis of mating system evolution in Leavenworthia has clarified that selection among individuals is likely a major force behind the recurrent evolution of selfing. Although inadequate pollination is appreciated as a factor favoring selfing, definitive ecological mechanisms that act to favor selfing are still not known and future work on the efficacy of pollinating bees and the effects of climate change is needed. Recent research has likely identified the SRK ortholog at the S-locus controlling self-incompatibility in Leavenworthia alabamica. Analyses of S-locus variation have revealed substantial S-allele diversity in outcrossing populations, with the recurrent fixation of mutations at the S-locus permitting the parallel evolution of selfing in this species. Although we appreciate some of the factors that may explain the evolution of selfing in this group, there is less known about the mechanisms underlying the widespread maintenance of outcrossing at the population and species levels. Studies in Leavenworthia have revealed that genetic diversity is lost over the long-term within selfing populations and leads to elevated population subdivision, but work is needed to determine why these genetic consequences of selfing cause lineages to become evolutionary dead ends.     

INBREEDING DEPRESSION IN TWO MIMULUS TAXA MEASURED BY MULTIGENERATIONAL CHANGES IN THE INBREEDING COEFFICIENT

In mixed-mating plant populations, one can estimate the relative fitness of selfed progeny w by measuring the inbreeding coefficient F and selfing rate s of adults of one generation, together with F of adults in the following generation (after selection). In the first application of this multigenerational method, we estimated F and s for adults over three consecutive generations in adjacent populations of two annual Mimulus taxa: the outbreeding M. guttatus and the inbreeding M. platycalyx. This gave estimates of w for the last two generations. Although average multilocus selfing rates were high in both taxa (0.63 in M. guttatus; 0.84 in M. platycalyx), the relative fitness of selfed progeny averaged only 0.19 in M. guttatus and 0.32 in M. platycalyx. An alternative estimator for w that incorporates biparental inbreeding gave even lower estimates of w. These values are significantly below the 0.5 threshold thought to favor selfing, and show that partially selfing populations can harbor substantial genetic load. In accordance with the purging hypothesis, the more highly selfing M. platycalyx showed marginally lower inbreeding depression than M. guttatus in both years (P = 0.08). Inbreeding depression and selfing rates also varied among years in concert among taxa. Several sources of bias are discussed, but computer simulations indicate it is unlikely that w is biased downwards by linkage of marker loci to load loci.     

Mating system and genetic differentiation inDasypyrum villosum (Poaceae) in Italy

Genetic differentiation at the morphological, isozyme, and DNA levels among sevenDasypyrum villosum (Poaceae, Triticeae, 2n = 14, VV genomes) populations from Italy was studied. A measure of the mating system was also obtained. Genetic diversity was mainly distributed within populations (90%) rather than among populations (10%), typical for most allogamous species. Interpopulation diversity, however, was greatest between the most geographically distant populations (about 750 km apart, 911 m altitude difference). The mating system was estimated to vary from 55 to 100% outcrossing. One population (I-16, Bomarzo) deviated from the others in its uniform early flowering habit and presence of isozyme alleles not found in other populations. It had genetic diversity similar to other populations for isozyme (Got-V2, Got-V3, andEst-VF) and ribosomal RNA (Nor-Vl) loci. This population is believed to be the product of a few migrant founder seeds and its unique characters point out that generalized population biology parameters of genetic diversity are not sufficient for describing species variation nor for developing conservation strategies.     

Adaptive genetic differentiation in life-history traits between populations of <Emphasis Type="Italic">Mimulus guttatus</Emphasis> with annual and perennial life-cycles

The optimal allocation to sexual and vegetative reproduction as well as the optimal values of other life-history characteristics such as phenology, growth and mating system are likely to depend on the life-cycle of the organism. I tested whether plants of Mimulus guttatus originating from temporarily wet populations where the species has an enforced annual life-cycle have higher allocation to sexual reproduction, lower allocation to vegetative reproduction, more rapid phenology, faster growth, and floral traits associated with a self-fertilizing mating system than plants from permanently wet populations where the species has a perennial life-cycle. I grew a total of 1377 plants originating from three populations with an annual life-cycle and 11 populations with a perennial life-cycle in a greenhouse under permanently and temporarily wet conditions. Plants of M. guttatus in permanently wet conditions had significantly more vegetative reproduction and tended to have a faster growth than plants in the temporarily wet conditions, indicating plasticity in these life-history traits. Plants from populations with an annual life-cycle invested significantly more in sexual reproduction and significantly less in vegetative reproduction than the ones from populations with a perennial life-cycle. Moreover, this study showed that plants originating from populations with an annual life-cycle have a significantly faster development and floral traits associated with autonomous self-fertilization. In conclusion, this study suggests that there has been adaptive evolution of life history traits of M. guttatus in response to natural watering conditions that determine the life span of the species.     

Divergent population structure and climate associations of a chromosomal inversion polymorphism across the Mimulus guttatus species complex

Chromosomal rearrangement polymorphisms are common and increasingly found to be associated with adaptive ecological divergence and speciation. Rearrangements, such as inversions, reduce recombination in heterozygous individuals and thus can protect favourable allelic combinations at linked loci, facilitating their spread in the presence of gene flow. Recently, we identified a chromosomal inversion polymorphism that contributes to ecological adaptation and reproductive isolation between annual and perennial ecotypes of the yellow monkeyflower, Mimulus guttatus. Here we evaluate the population genetic structure of this inverted region in comparison with the collinear regions of the genome across the M. guttatus species complex. We tested whether annual and perennial M. guttatus exhibit different patterns of divergence for loci in the inverted and noninverted regions of the genome. We then evaluated whether there are contrasting climate associations with these genomic regions through redundancy analysis. We found that the inversion exhibits broadly different patterns of divergence among annual and perennial M. guttatus and is associated with environmental variation across population accessions. This study is the first widespread population genetic survey of the diversity of the M. guttatus species complex. Our findings contribute to a greater understanding of morphological, ecological, and genetic evolutionary divergence across this highly diverse group of closely related ecotypes and species. Finally, understanding species relationships among M. guttatus sp. has hitherto been stymied by accumulated evidence of substantial gene flow among populations as well as designated species. Nevertheless, our results shed light on these relationships and provide insight into adaptation in life history traits within the complex.     

Out in the cold: genetic variation of Nothofagus pumilio (Nothofagaceae) provides evidence for latitudinally distinct evolutionary histories in austral South America

Nothofagus pumilio is the dominant and almost ubiquitous tree species in mountainous environments of temperate South America. We used two types of molecular markers (cpDNA and isozymes) to evaluate the effects of the Paleogene paleogeography of Patagonia and more recent climatic oscillations of the Neogene on such cold‐tolerant species’ genetic makeup. Phylogeographic analysis on sequences of three cpDNA non‐coding regions at 85 populations yielded two latitudinally disjunct monophyletic clades north and south of c. 42°S containing 11 and three haplotypes, respectively. This indicates a long‐lasting vicariant event due to the presence of an extended open paleobasin at mid latitudes of Patagonia. Also distribution patterns of cpDNA haplotypes suggest regional spread following stepping‐stone models using pre‐Cenozoic mountains as corridors. Comparable genetic diversity measured along 41 sampled populations using seven polymorphic isozyme loci provides evidence of local persistence and spread from multiple ice‐free locations. In addition, significantly higher heterozygosity and allelic richness at high latitudes, i.e. in areas of larger glacial extent, suggest survival in large and isolated refugia. While, higher cpDNA diversity in lower latitudes reflects the complex orogeny that historically isolated northern populations, lower isozyme diversity and reduced F_ST values provide evidence of local glacial survival in numerous small locales. Therefore, current genetic structure of N. pumilio is the result of regional processes which took place during the Tertiary that were enhanced by contemporary local effects of drift and isolation in response to Quaternary climatic cycles.     

Conservation genetics of Sinai’s remnant populations of Moringa peregrina, an economically valuable medicinal plant

Moringa peregrina is an economically valuable tree of Egyptian deserts. It is used medicinally, provides a highly nutritious supplement to Bedouin diets, provides fodder for livestock, and is used for fire wood. M. peregrina seeds have been a source of high-quality oil for cosmetics and perfumes since antiquity. Due to unmanaged grazing and over-collection, M. peregrina has become one of the most endangered tree species in the Egyptian desert ecosystem. A long-term conservation program is urgently needed to maintain or increase the number and size of M. peregrina populations. Ten populations harboring a total of 130 adult M. peregrina were sampled from three disjunct Wadis in South Sinai (W. Me’ar, W. Fieran and W. Zaghra). Open-pollinated seedlings were electrophoretically analyzed to address two basic questions: (1) how is genetic diversity distributed within and among populations within these three Wadis; and (2) what is the mating system of this species. M. peregrina has a mixed mating system with a selfing rate up to 16% and has limited genetic diversity within and significant genetic differentiation among its populations, the majority of which occurs among Wadis. Direct protection is urgently needed to decrease genetic deterioration within M. peregrina populations and to improve their ability to maintain or improve their population numbers. The priority of in situ conservation should be to conserve a few large well-distributed populations representing different Wadis. Ex situ germplasm collections should be made across the species’ range to ensure a representative sample of its genetic variation. Seed orchards designed to maximize cross-fertilization among unrelated individuals should be established to generate propagules to supplement natural populations.     

Patterns of nucleotide diversity in two species of Mimulus are affected by mating system and asymmetric introgression

The evolutionary transition from outcrossing to self-fertilization has far-reaching implications for patterns of intraspecific genetic diversity and the potential for speciation. Using DNA sequence variation at two nuclear loci, we examined the divergence history of two closely related species of Mimulus. To investigate the effects of mating system and introgressive hybridization on the outcrossing M. guttatus and the selfing M. nasutus, we inspected nucleotide diversity within and between natural populations spanning the species' geographic ranges. High sequence similarity among populations of the selfing M. nasutus points to a single evolutionary origin for the species. Consistent with their distinct mating systems, all genetic variation in M. nasutus is distributed among populations, whereas M. guttatus exhibits appreciable levels of nucleotide diversity within populations. Silent genetic diversity is extensive in M. guttatus (mean theta(sil)/site = 0.077) and greatly exceeds the predicted twofold elevation in neutral variation for outcrossers relative to selfers. The finding of several M. guttatus sequences that share complete identity with sequences from M. nasutus suggests that recent asymmetric introgression may have occurred. We argue that exceptionally high nucleotide diversity in M. guttatus is consistent with a long-term history of directional introgression from M. nasutus to M. guttatus throughout the divergence of these two species.     

Levels of DNA polymorphism vary with mating system in the nematode genus caenorhabditis

Self-fertilizing species often harbor less genetic variation than cross-fertilizing species, and at least four different models have been proposed to explain this trend. To investigate further the relationship between mating system and genetic variation, levels of DNA sequence polymorphism were compared among three closely related species in the genus Caenorhabditis: two self-fertilizing species, Caenorhabditis elegans and C. briggsae, and one cross-fertilizing species, C. remanei. As expected, estimates of silent site nucleotide diversity were lower in the two self-fertilizing species. For the mitochondrial genome, diversity in the selfing species averaged 42% of diversity in C. remanei. Interestingly, the reduction in genetic variation was much greater for the nuclear than for the mitochondrial genome. For two nuclear genes, diversity in the selfing species averaged 6 and 13% of diversity in C. remanei. We argue that either population bottlenecks or the repeated action of natural selection, coupled with high levels of selfing, are likely to explain the observed reductions in species-wide genetic diversity.     

Genetic comparison between Pinus sylvestris and P. mugo using isozymes and chloroplast DNA

Pinus sylvestris and P. mugo populations from Poland and Czechoslovakia were compared using genetic variability at isozyme markers, chloroplast DNA variation, and mating system measurements. Two isozyme loci were found to differ between the species. P. mugo was as variable at isozyme loci as P. sylvestris. Diagnostic cpDNA fragments were found using the restriction enzyme Bcl-I. Populations that were morphologically classified as hybrids were found to be pure species, based both on isozyme and cpDNA results.