Is embryo abortion a post‐zygotic barrier to gene flow between Littorina ecotypes?

Genetic incompatibilities contribute to reproductive isolation between many diverging populations, but it is still unclear to what extent they play a role if divergence happens with gene flow. In contact zones between the "Crab" and "Wave" ecotypes of the snail Littorina saxatilis, divergent selection forms strong barriers to gene flow, while the role of post‐zygotic barriers due to selection against hybrids remains unclear. High embryo abortion rates in this species could indicate the presence of such barriers. Post‐zygotic barriers might include genetic incompatibilities (e.g. Dobzhansky–Muller incompatibilities) but also maladaptation, both expected to be most pronounced in contact zones. In addition, embryo abortion might reflect physiological stress on females and embryos independent of any genetic stress. We examined all embryos of >500 females sampled outside and inside contact zones of three populations in Sweden. Females' clutch size ranged from 0 to 1,011 embryos (mean 130 ± 123), and abortion rates varied between 0% and 100% (mean 12%). We described female genotypes by using a hybrid index based on hundreds of SNPs differentiated between ecotypes with which we characterized female genotypes. We also calculated female SNP heterozygosity and inversion karyotype. Clutch size did not vary with female hybrid index, and abortion rates were only weakly related to hybrid index in two sites but not at all in a third site. No additional variation in abortion rate was explained by female SNP heterozygosity, but increased female inversion heterozygosity added slightly to increased abortion. Our results show only weak and probably biologically insignificant post‐zygotic barriers contributing to ecotype divergence, and the high and variable abortion rates were marginally, if at all, explained by hybrid index of females.     

Incipient post‐zygotic barrier in a model system of ecological speciation with gene flow

The role of post‐zygotic isolation in nonallopatric ecological speciation is still mostly unknown and information on the nature and strength of these barriers in well‐known speciation models is essential for a deeper understanding of such processes. The Galician ecotypes of the marine snail Littorina saxatilis represent one of the best studied cases of nonallopatric ecological speciation. Here, we test the existence of incipient post‐zygotic isolation by comparing the fertility of male hybrids with that of both pure forms [ridged and banded (RB) and smooth and unbanded (SU) ecotypes]. We analysed the degree of sperm DNA fragmentation (SDF) of individuals morphologically classified as RB, SU and hybrids, sampled from two locations. SDF analyses were chosen to study sperm quality because, in other animal species, SDF rates correlate with important parameters for speciation research, such as fertilization and abortion rates and viability of adult progeny. In the present work, hybrids showed significantly higher SDF rates than RB and SU males in one location and significantly higher variances in both locations. These results suggest the existence of an incipient post‐zygotic barrier, the strength of which may vary across the Galician shore, and highlight the potential of SDF analyses for speciation research.     

Evolution of pre‐zygotic and post‐zygotic barriers to gene flow among three cryptic species within the Anastrepha fraterculus complex

Tropical tephritids are ideally suited for studies on population divergence and speciation because they include species groups undergoing rapid radiation, in which morphologically cryptic species and sister species are abundant. The fraterculus species group in the Neotropical genus Anastrepha is a case in point, as it is composed of a complex of up to seven A. fraterculus morphotypes proposed to be cryptic species. Here, we document pre‐ and post‐zygotic barriers to gene flow among adults of the Mexican A. fraterculus morphotype and three populations (Argentina, Brazil, and Peru) belonging to two separate morphotypes (Brazilian 1 and Peruvian). We unveiled three forms of pre‐zygotic reproductive isolation resulting in strong assortative mating. In field cages, free‐ranging male and female A. fraterculus displayed a strong tendency to form couples with members of the opposite sex belonging to their own morphotype, suggesting that male pheromone emission, courtship displays, or both intervene in shaping female choice before actual contact and coupling. In addition, males and females of the Peruvian morphotype became receptive and mated significantly later than adults of the Mexican and Brazilian 1 morphotypes. After contact, Mexican females exhibited greater mating discrimination than males when facing adults of the opposite sex belonging to either the Peruvian or the Brazilian 1 morphotype as evidenced by vigorous resistance to penetration once they had been forcefully mounted by heterotypic males. Forced copulations resulted in production of F1 hybrids that were either less viable (and partially fertile) than parental crosses or even sterile. Our results suggest that the Mexican morphotype is a distinct biological entity and that pre‐zygotic reproductive isolation through divergence in courtship or male‐produced pheromone and other mechanisms appear to evolve faster than post‐zygotic isolation in the fraterculus species group.     

Quantification of absolute coronary flow reserve and relative fractional flow reserve in a swine animal model using angiographic image data

Coronary flow reserve (CFR) and fractional flow reserve (FFR) are important physiological indexes for coronary disease. The purpose of this study was to validate the CFR and FFR measurement techniques using only angiographic image data. Fifteen swine were instrumented with an ultrasound flow probe on the left anterior descending artery (LAD). Microspheres were gradually injected into the LAD to create microvascular disruption. An occluder was used to produce stenosis. Contrast material injections were made into the left coronary artery during image acquisition. Volumetric blood flow from the flow probe (Q(q)) was continuously recorded. Angiography-based blood flow (Q(a)) was calculated by using a time-density curve based on the first-pass analysis technique. Flow probe-based CFR (CFR(q)) and angiography-based CFR (CFR(a)) were calculated as the ratio of hyperemic to baseline flow using Q(q) and Q(a), respectively. Relative angiographic FFR (relative FFR(a)) was calculated as the ratio of the normalized Q(a) in LAD to the left circumflex artery (LC(X)) during hyperemia. Flow probe-based FFR (FFR(q)) was measured from the ratio of hyperemic flow with and without disease. CFR(a) showed a strong correlation with the gold standard CFR(q) (CFR(a) = 0.91 CFR(q) + 0.30; r = 0.90; P < 0.0001). Relative FFR(a) correlated linearly with FFR(q) (relative FFR(a) = 0.86 FFR(q) + 0.05; r = 0.90; P < 0.0001). The quantification of CFR and relative FFR(a) using angiographic image data was validated in a swine model. This angiographic technique can potentially be used for coronary physiological assessment during routine cardiac catheterization.     

Regulation of zygotic gene activation in the mouse

Zygotic gene activation (ZGA) is the critical event that governs the transition from maternal to embryonic control of development. In the mouse, ZGA occurs during the 2-cell stage and appears to be regulated by the time following fertilization, i.e. a zygotic clock, rather than by progression through the first cell cycle. The onset of ZGA must depend on maternally inherited proteins, and post-translational modification of these maternally derived proteins is likely to play a role in ZGA. Consistent with this prediction is that protein phosphorylation catalyzed by the cAMP-dependent protein kinase is involved in ZGA and that protein synthesis is not required for ZGA. Recent results suggest that ZGA may occur earlier than previously thought, i.e. not during the 2-cell stage, but rather in G₂ of the 1-cell embryo. Thus ZGA may comprise a period of minor gene activation in the 1-cell embryo that is followed by a period of major gene activation in the 2-cell embryo. Following ZGA, the expression of constitutively activated genes may require an enhancer.     

A sea water barrier to coral gene flow

Land is not the only barrier to dispersal encountered by marine organisms. For sedentary shallow water species, there is an additional, marine barrier, 5000 km of uninterrupted deep‐water stretch between the central and the eastern Pacific. This expanse of water, known as the ‘Eastern Pacific Barrier’, has been separating faunas of the two oceanic regions since the beginning of the Cenozoic. Species with larvae that cannot stay in the plankton for the time it takes to cross between the two sides have been evolving independently. That the eastern Pacific does not share species with the rest of the Pacific was obvious to naturalists two centuries ago (Darwin 1860). Yet, this rule has exceptions. A small minority of species are known to straddle the Eastern Pacific Barrier. One such exception is the scleractinian coral Porites lobata (Fig.  1 ). This species is spread widely throughout the Indo‐Pacific, where it is one of the major reef‐builders, but it is also encountered in the eastern Pacific. Are eastern and central Pacific populations of this coral connected by gene flow? In this issue of Molecular Ecology, Baums et al. (2012) use microsatellite data to answer this question. They show that P. lobata populations in the eastern Pacific are cut off from genetic influx from the rest of the Pacific. Populations within each of the two oceanic regions are genetically connected (though those in the Hawaiian islands are also isolated). Significantly, the population in the Clipperton Atoll, the westernmost island in the eastern Pacific, genetically groups with populations from the central Pacific, suggesting that crossing the Eastern Pacific Barrier by P. lobata propagules does occasionally occur.     

Mating system as a barrier to gene flow

Understanding mating system as one of reproductive isolating barriers remains important although this barrier is classified in a different sense from behavioral, ecological, and mechanical isolating barriers. Selfing enhances incipient speciation while outcrossing facilitates species integrity. Here, I study how mating system affects gene exchanges between genetically diverging species in a hybrid zone. Results show that a predominant selfing species has a greater barrier to selective gene flow than does a predominant outcrossing species. Barrier to neutral gene flow convexly changes with the selfing rate due to linkage disequilibrium, with a maximum at around intermediate selfing rate. Asymmetric transient or steady‐state barriers to neutral gene flow occur between two sides of a hybrid zone when the neutral gene is affected by its linked selective gene whose alternative alleles are adaptive to heterogeneous habitats. Selfing interacts with both a physical barrier and a density‐dependent ecological regulation (a logarithmic model) to strengthen the barriers to neutral and selective gene flow. This theory helps to interpret incipient speciation driven by selfing or to explain the asymmetric gene flow or unequal genomic mixtures between closely related species caused by their asymmetric mating systems in natural hybrid zones.     

Immigrant inviability produces a strong barrier to gene flow between parapatric ecotypes of Senecio lautus

Speciation proceeds when gene exchange is prevented between populations. Determining the different barriers preventing gene flow can therefore give insights into the factors driving and maintaining species boundaries. These reproductive barriers may result from intrinsic genetic incompatibilities between populations, from extrinsic environmental differences between populations, or a combination of both mechanisms. We investigated the potential barriers to gene exchange between three adjacent ecotypes of an Australian wildflower to determine the strength of individual barriers and the degree of overall isolation between populations. We found almost complete isolation between the three populations mainly due to premating extrinsic barriers. Intrinsic genetic barriers were weak and variable among populations. There were asymmetries in some intrinsic barriers due to the origin of cytoplasm in hybrids. Overall, these results suggest that reproductive isolation between these three populations is almost complete despite the absence of geographic barriers, and that the main drivers of this isolation are ecologically based, consistent with the mechanisms underlying ecological speciation.     

Hybrid zones and the genetic architecture of a barrier to gene flow between two sunflower species.

Genetic analyses of reproductive barriers represent one of the few methods by which theories of speciation can be tested. However, genetic study is often restricted to model organisms that have short generation times and are easily propagated in the laboratory. Replicate hybrid zones with a diversity of recombinant genotypes of varying age offer increased resolution for genetic mapping experiments and expand the pool of organisms amenable to genetic study. Using 88 markers distributed across 17 chromosomes, we analyze the introgression of chromosomal segments of Helianthus petiolaris into H. annuus in three natural hybrid zones. Introgression was significantly reduced relative to neutral expectations for 26 chromosomal segments, suggesting that each segment contains one or more factors that contribute to isolation. Pollen sterility is significantly associated with 16 of these 26 segments, providing a straightforward explanation of why this subset of blocks is disadvantageous in hybrids. In addition, comparison of rates of introgression across colinear vs. rearranged chromosomes indicates that close to 50% of the barrier to introgression is due to chromosomal rearrangements. These results demonstrate the utility of hybrid zones for identifying factors contributing to isolation and verify the prediction of increased resolution relative to controlled crosses.     

Estimating exotic gene flow into native pine stands: zygotic vs. gametic components

Monitoring contemporary gene flow from widespread exotic plantations is becoming an important problem in forest conservation genetics. In plants, where both seed and pollen disperse, three components of exotic gene flow with potentially unequal consequences should be, but have not been, explicitly distinguished: zygotic, male gametic and female gametic. Building on a previous model for estimating contemporary rates of zygotic and male gametic gene flow among plant populations, we present here an approach that additionally estimates the third (female gametic) gene flow component, based on a combination of uni‐ and biparentally inherited markers. Using this method and a combined set of chloroplast and nuclear microsatellites, we estimate gene flow rates from exotic plantations into two Iberian relict stands of maritime pine (Pinus pinaster) and Scots pine (Pinus sylvestris). Results show neither zygotic nor female gametic gene flow but moderate (6–8%) male gametic introgression for both species, implying significant dispersal of pollen, but not of seeds, from exotic plantations into native stands shortly after introduced trees reached reproductive maturity. Numerical simulation results suggest that the model yields reasonably accurate estimates for our empirical data sets, especially for larger samples. We discuss conservation management implications of observed levels of exposure to nonlocal genes and identify research needs to determine potentially associated hazards. Our approach should be useful for plant ecologists and ecosystem managers interested in the vectors of contemporary genetic connectivity among discrete plant populations.     

Massive gene flow across the world's most potent marine biogeographic barrier

The ''Eastern Pacific Barrier'' (EPB), 5400 km of uninterrupted deep water between the central and eastern Pacific, constitutes the greatest marine obstacle to the dispersal of shallow-water organisms. However, some species are found on both sides of the EPB. These ''transpacific'' species are considered by ''dispersal'' biogeographers as evidence of invasions through the barrier. ''Vicariance'' biogeographers, on the other hand, think that transpacific species are morphologically conservative remnants of previously continuous distributions. We compared nucleotide sequences in a 642 bp region of mitochondrial DNA, and electrophoretically detected alleles in 17 enzymatic loci of central and eastern Pacific populations of Echinothrix diadema, an Indo-Pacific sea urchin recently reported from the eastern Pacific. Both types of molecules produced clear evidence of massive, recent gene flow across the EPB. Thus, rather than being isolated relicts of Tethyan distributions, conspecific populations from the eastern and central Pacific are genetically connected. Though the EPB is biogeographically important as a cause of speciation in many groups, it allows genetic connections in others, possibly through larval transport during El Niño events.     

The Himalayas as a directional barrier to gene flow

High-resolution Y-chromosome haplogroup analyses coupled with Y-short tandem repeat (STR) haplotypes were used to (1) investigate the genetic affinities of three populations from Nepal--including Newar, Tamang, and people from cosmopolitan Kathmandu (referred to as "Kathmandu" subsequently)--as well as a collection from Tibet and (2) evaluate whether the Himalayan mountain range represents a geographic barrier for gene flow between the Tibetan plateau and the South Asian subcontinent. The results suggest that the Tibetans and Nepalese are in part descendants of Tibeto-Burman-speaking groups originating from Northeast Asia. All four populations are represented predominantly by haplogroup O3a5-M134-derived chromosomes, whose Y-STR-based age (+/-SE) was estimated at 8.1+/-2.9 thousand years ago (KYA), more recent than its Southeast Asian counterpart. The most pronounced difference between the two regions is reflected in the opposing high-frequency distributions of haplogroups D in Tibet and R in Nepal. With the exception of Tamang, both Newar and Kathmandu exhibit considerable similarities to the Indian Y-haplogroup distribution, particularly in their haplogroup R and H composition. These results indicate gene flow from the Indian subcontinent and, in the case of haplogroup R, from Eurasia as well, a conclusion that is also supported by the admixture analysis. In contrast, whereas haplogroup D is completely absent in Nepal, it accounts for 50.6% of the Tibetan Y-chromosome gene pool. Coalescent analyses suggest that the expansion of haplogroup D derivatives--namely, D1-M15 and D3-P47 in Tibet--involved two different demographic events (5.1+/-1.8 and 11.3+/-3.7 KYA, respectively) that are more recent than those of D2-M55 representatives common in Japan. Low frequencies, relative to Nepal, of haplogroup J and R lineages in Tibet are also consistent with restricted gene flow from the subcontinent. Yet the presence of haplogroup O3a5-M134 representatives in Nepal indicates that the Himalayas have been permeable to dispersals from the east. These genetic patterns suggest that this cordillera has been a biased bidirectional barrier.     

Quantification of diel vertical migration by micronektonic taxa in the northeast Atlantic

Analyses of day/night changes in the bathymetric distribution of micronektonic biomass at 16 stations in the northeastern Atlantic, sampled between 1978 and 1994, provided quantitative estimates of the organic carbon fluxes associated with diel vertical migration of individual taxa of micronekton. Gelatinous taxa contributed 50–80% of the integrated standing crop by volume but, apart from tunicates, contributed relatively little to the active migratory fluxes when expressed in terms of carbon. Total micronektonic migratory fluxes into the upper 200 m ranged from 12.5 to 58 mgC per m². At 15 stations, fish and pteropods provided 50–80% of the fluxes into the upper 100, 200 and 400 m. At one station, tunicates (pyrosomes) contributed substantially. Wherever tunicates or the medusa Pelagia were swarming, migrations by other taxa appear to be suppressed. The mean proportions of the stock (in terms of biomass) of each of the dominant migratory taxa entering and leaving the upper 100 m were 23% for tunicates, 18% for fish, 22% for pteropods, 8% for decapod crustaceans and 23% for euphausiids. The maximum proportions for these five taxa were 90%, 60%, 75%, 25% and 75%, respectively. Similar estimates of the mean fluxes into and out of the upper 400 m were generally higher: 19% for tunicates, 39% for fish, 28% for pteropods, 49% for decapods and 55% for euphausiids; the respective maxima were 99%, 74%, 99%, 72% and 91%. It is estimated for fish that if these migrations occur throughout the year, they will result in an active carbon export (both POC and DOC) from the wind-mixed layer and immediate sub-thermocline depths of about 500% of the mean annual standing stock. If this estimate can be extended to other taxa, then the material fluxes resulting from these active migrations will be quantitatively similar to those resulting from the deposition of phytodetritus at temperate latitudes.     

Pervasive gene flow across critical habitat for four narrowly endemic,sympatric taxa

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