Genetic signature of recent glaciation on populations of a near-shore marine fish species (Syngnathus leptorhynchus)

Continental glaciation has played a major role in shaping the present-day phylogeography of freshwater and terrestrial species in the Northern Hemisphere. Recent work suggests that coastal glaciation during ice ages may have also had a significant impact on marine species. The bay pipefish, Syngnathus leptorhynchus , is a near-shore Pacific coast fish species with an exceptionally wide latitudinal distribution, ranging from Bahia Santa Maria, Baja California to Prince William Sound, Alaska. Survey data indicate that S. leptorhynchus is experiencing a range expansion at the northern limit of its range, consistent with colonization from southern populations. The present study uses six novel microsatellite markers and mitochondrial DNA (mtDNA) sequence data to study the present-day population genetic structure of four coastal populations of S. leptorhynchus . Deficits in mtDNA and nuclear DNA diversity in northern populations from regions glaciated during the last glacial maximum (LGM) [ c . 18 000 years before present ( bp )] suggest that these populations were effected by glacial events. Direct estimates of population divergence times derived from both isolation and isolation-with-migration models of evolution are also consistent with a postglacial phylogenetic history of populations north of the LGM. Sequence data further indicate that a population at the southern end of the species range has been separated from the three northern populations since long before the last interglacial event ( c . 130 000 years bp ), suggesting that topographical features along the Pacific coast may maintain population separation in regions unimpacted by coastal glaciation.     

Fecundity selection predicts Bergmann's rule in syngnathid fishes

The study of latitudinal increases in organismal body size (Bergmann's rule) predates even Darwin's evolutionary theory. While research has long concentrated on identifying general evolutionary explanations for this phenomenon, recent work suggests that different factors operating on local evolutionary timescales may be the cause of this widespread trend. Bergmann's rule explains body size variation in a diversity of warm-blooded organisms and there is increasing evidence that Bergmann's rule is also widespread in ectotherms. Bergmann's rule acts differentially in species of the Syngnathidae, a family of teleost fishes noted for extreme adaptations for male parental care. While variation in body size of polygamous Syngnathus pipefish is consistent with Bergmann's rule, body size is uncorrelated with latitude in monogamous Hippocampus seahorses. A study of populations of Syngnathus leptorhynchus along a natural latitudinal and thermal gradient indicates that increases in body size with latitude maintain the potential reproductive rate of males despite significant decreases in ambient temperatures. Polygyny is necessary in order to maximize male reproductive success in S. leptorhynchus , suggesting a possible a link between fecundity selection and Bergmann's rule in this species.     

Geographical variation in the mating system of the dusky pipefish (Syngnathus floridae)

Differences among populations in the intensity of sexual selection resulting from distinct genetic mating systems can lead to divergent morphological evolution and speciation. However, little is known about how genetic mating systems vary between populations and what factors may contribute to this variation. In this study, we compare the genetic mating systems of two geographically distinct populations of the dusky pipefish (Syngnathus floridae), a species characterized by polygynandry and male pregnancy, from the Atlantic Coast of Virginia and the Gulf Coast of Florida. Our results revealed significant interpopulation variation in mating and reproductive success. Estimates of the opportunity for selection (I), the opportunity for sexual selection (I(s)) and the Bateman gradient (beta(ss)) were higher among males in the Florida population than in the Virginia population, suggesting that sexual selection on males is stronger in the Florida population. The Virginia population is larger and denser than the Florida population, suggesting that population demographics may be one of many causal factors shaping interpopulational mating patterns. This study also provides evidence that the adult sex ratio, operational sex ratio, population density and genetic mating system of S. floridae may be temporally stable over timescales of a month in the Florida population. Overall, our results show that this species is a good model for the study of mating system variation in nature and that Bateman's principles may be a useful technique for the quantitative comparison of mating systems between populations.     

Environmental, demographic, and genetic mating system variation among five geographically distinct dusky pipefish (Syngnathus floridae) populations

Genetic mating systems are expected to vary among and within populations in response to environmental and demographic factors. Despite the fact that mating system variation theoretically can have profound effects on important evolutionary processes such as sexual selection, extensive intraspecific surveys of geographical variation in mating systems are rare. We used microsatellite markers to characterize genetic mating systems of dusky pipefish, Syngnathus floridae , from five populations distributed from the mid-Atlantic Coast to the Western Gulf of Mexico. We also measured a number of environmental and demographic variables to examine correlations between the ecological setting and mating behaviour. Our results show that dusky pipefish are polygynandrous throughout their USA distribution, but they exhibit a wide range of quantitative variation in male mating behaviour. In addition, these five populations varied substantially with respect to environmental and demographic variables, and some of these were significantly correlated with aspects of the genetic mating system. While causal relationships cannot be firmly diagnosed from this type of comparative study, our results do identify several ecological factors, such as water temperature, adult sex ratio, and seagrass biomass, which should be considered in future experimental and comparative work. Overall, this study confirms the expectation that geographical variation in mating systems is widespread and shows that the dusky pipefish is an excellent model for continued research into the factors affecting mating systems in nature.     

Opening Pandora's box: comparative studies of genetic mating systems reveal reproductive complexity

Genetic analyses of realized reproductive success have fundamentally changed our understanding of mating behaviour in natural systems. While behavioural ecologists have long been interested in what factors influence mating behaviour, early studies were limited to direct observations of matings and thus provided an incomplete picture of reproductive activity. Genetic assessments of parentage have revolutionized the study of reproductive behaviour, revealing that many individuals engage in extra‐pair copulations ( Griffith et al. 2002 ) and that social mating partners frequently invest substantial resources into raising offspring that are unrelated to one or both of them ( Avise et al. 2002 ). While these findings have changed the way we think about reproductive behaviour, most investigations of genetic parentage have been restricted to single populations at a single point in time, obscuring spatial and/or temporal variation in mating behaviour and limiting our ability to determine how environmental changes can lead to shifts in mating strategies. In this issue of Molecular Ecology, Mobley & Jones (2009) compare genetic mating behaviour across five populations of Syngnathus floridae ( Fig. 1 ), a widespread species of pipefish distributed along the Gulf‐ and Atlantic Coasts of North America. The authors document how genetic mating behaviour varies across space in S. floridae and identify correlations between reproductive variation and particular ecological characteristics. Mobley & Jones’ paper is one of an increasing number of studies which address the question of how ecological variables influence mating behaviour, and highlights how our understanding of mating system variation and evolution is likely to expand through the wider application of high‐throughput parentage assessment in a comparative context.

Figure 1 Open in figure viewer PowerPoint A pregnant male dusky pipefish (Syngnathus floridae) in its natural habitat. Photo credit: Joe O’Hop.     

POLYGYNANDRY IN THE DUSKY PIPEFISH SYNGNATHUS FLORIDAE REVEALED BY MICROSATELLITE DNA MARKERS

In the dusky pipefish Syngnathus floridae, like other species in the family Syngnathidae, ‘pregnant’ males provide all post-zygotic care. Male pregnancy has interesting implications for sexual selection theory and the evolution of mating systems. Here, we employ microsatellite markers to describe the genetic mating system of S. floridae, compare the outcome with a previous report of genetic polyandry for the Gulf pipefish S. scovelli, and consider possible associations between the mating system and degree of sexual dimorphism in these species. Twenty-two pregnant male dusky pipefish from one locale in the northern Gulf of Mexico were analyzed genetically, together with subsamples of 42 embryos from each male's brood pouch. Adult females also were assayed. The genotypes observed in these samples document that cuckoldry by males did not occur; males often receive eggs from multiple females during the course of a pregnancy (six males had one mate each, 13 had two mates, and three had three mates); embryos from different females are segregated spatially within a male's brood pouch; and a female's clutch of eggs often is divided among more than one male. Thus, the genetic mating system of the dusky pipefish is best described as polygynandrous. The genetic results for S. floridae and S. scovelli are consistent with a simple model of sexual selection which predicts that for sex role-reversed organisms, species with greater degrees of sexual dimorphism are more highly polyandrous.     

The relationship between female body size and egg size in pipefishes

Comparing five species of pipefish, egg size was significantly larger in species with brood pouches (Syngnathus typhle, Syngnathus acus and Syngnathus rostellatus) than in species without brood pouches (Entelurus aequoreus and Nerophis ophidion). Egg size correlated positively with female body size in species with brood pouches, but was similar across female sizes in the species lacking pouches. These results may reflect differences in offspring competition as a consequence of variable offspring relatedness within a brood, due to the mating systems adopted by the different species and the presence or absence of a brood pouch.     

Identification of a uniquely southern African clade of coastal pipefishes Syngnathus spp

The taxonomic status of two southern African coastal pipefish species, Syngnathus temminckii and Syngnathus watermeyeri, was investigated using a combination of morphological and genetic data. Morphological data showed that S. temminckii is distinct from the broadly distributed European pipefish Syngnathus acus, and a molecular phylogeny reconstructed using mitochondrial DNA recovered S. temminckii and S. watermeyeri as sister taxa. The southern African species share an evolutionary origin with north‐eastern Atlantic Ocean and Mediterranean Sea species, including S. acus. These data support the existence of a distinct southern African clade of Syngnathus pipefishes that has diverged in situ to form the two species present in the region today.     

The ovarian structure and mode of egg production in two polygamous pipefishes: a link to mating pattern

In this study, the ovarian structure and mode of egg production were examined in two pipefishes, the broad-nosed pipefish Syngnathus typhle and the straight-nosed pipefish Nerophis ophidion, which show different types of polygamous mating patterns. Syngnathus typhle showed an ovary with one germinal ridge and asynchronous egg production, corresponding to previous findings in other polygamous Syngnathus pipefishes. In contrast, the ovary of N. ophidion had two germinal ridges and eggs were produced synchronously in groups, similar to what has been observed in monogamous syngnathids. The egg production of N. ophidion, however, is clearly distinguished from that of monogamous syngnathids by the additional egg production after an ovulation. It is suggested that the differences in female mating strategies result from the difference in egg production process and that this is related to the difference in mating pattern between these two polygamous species.     

Hosts are ahead in a marine host-parasite coevolutionary arms race: innate immune system adaptation in pipefish syngnathus typhle against Vibrio phylotypes

Microparasites have a higher evolutionary potential than their hosts due to an increased mutation rate and a shorter generation time that usually results in parasites being locally adapted to their sympatric hosts. This pattern may not apply to generalist pathogens as adaptation to sympatric host genotypes is disadvantageous due to a narrowing of the host range, in particular under strong gene flow among host populations. Under this scenario, we predict that the immune defense of hosts reveals adaptation to locally common pathogen phylotypes. This was tested in four host populations of the pipefish Syngnathus typhle and associated bacteria of the genus Vibrio. We investigated the population divergence among host and bacteria populations and verified that gene flow is higher among host populations than among parasite populations. Next, we experimentally assessed the strength of innate immune defense of pipefish hosts using in vitro assays that measured antimicrobial activity of blood plasma against sympatric and allopatric Vibrio phylotypes. Pipefish plasma displays stronger antimicrobial activity against sympatric Vibrio phylotypes compared to allopatric ones. This suggests that host defense is genetically adapted against local bacteria with a broad and unspecialized host spectrum, a situation that is typical for marine systems with weak host population structure.     

The Role of Courtship Behavior and Size in Mate Preference in the Sex‐Role‐Reversed Gulf Pipefish,Syngnathus scovelli

In sex‐role‐reversed species, sexual selection acts more strongly on females than on males, a situation that can result in the evolution of secondary sexual traits in females and strong mating preferences in males. While some research exploring mating preferences in sex‐role‐reversed species has been conducted, overall, this topic remains relatively unexplored. The Gulf pipefish, Syngnathus scovelli, is a highly polyandrous pipefish species. Sexual selection is significantly stronger in females than in males, which has led to the evolution of both morphological and behavioral female secondary sexual traits. However, because males gestate the offspring in specialized pouches and make a substantial investment in embryos during development, females may also benefit from being choosy. The goal of this study was to examine both male and female mating preferences in this species. We found that male mating preference was significantly associated with female courtship behavior. Larger females were also able to maintain these behaviors for longer intervals than smaller females. No evidence of female mating preference in regard to male size was observed but the data suggest that male behaviors may be providing positive reinforcement to courting females. This research provides further insight into how mate preferences vary among sex‐role‐reversed species.     

Mate quality influences multiple maternity in the sex-role-reversed pipefish Syngnathus typhle

In the pipefish Syngnathus typhle , pregnant males provide all parental care. Females are able to produce more eggs than males can brood, and consequently females compete more intensely for mates than do males, a phenomenon defined as sex-role reversal. As the genetic mating system influences the operation of sexual selection, we investigate variation in one phenotypic component of mate quality, female body size, as a possible proximate influence on mating system variation in S. typhle . Breeding trials were employed, each consisting of a single receptive male with four adult females. In each replicate, a focal male was paired either with a set of small or with a set of large females. Males were allowed to mate freely, and after several weeks of brood development, maternity of the progeny was resolved using three microsatellite loci. Males with access either to small or to large females successfully mated with a mean of 2.1 or 1.3 females, respectively, a significant difference. Results indicate that variation in female size can affect the mating system and thereby influence sexual selection in pipefish. Thus, the high rate of multiple mating by S. typhle males in the wild may be explained in part by the extensive size variation in naturally occurring, sexually mature females.     

Hybridization dynamics between sympatric species of trout: loss of reproductive isolation

Although reinforcement should enhance reproductive barriers in sympatric species, sympatric trout species do hybridize. Using mitochondrial and nuclear species markers, we investigated hybridization directionality, hybrid mating biases, and selection against hybrids in 13 sympatric cut-throat and rainbow trout populations on Vancouver Island, Canada. Approximately 50% of the genotyped fish were hybrid (F1 or higher-order) and populations ranged from very recent (all F1 hybrids) to extremely advanced higher-order hybridization. Overall, interbreeding was reciprocal, although some populations showed directional hybridization. Pronounced cytonuclear disequilibrium in post-F1 hybrids indicated a remarkable mating bias not previously reported, which is most likely because of behavioural reproductive preferences. Selection against hybrids was observed in only two populations, indicative of extrinsic selection. Two populations were 'hybrid swarms', with a complete loss of reproductive isolation. The complex hybridization dynamics in this system represent a valuable natural experiment of the genetic and evolutionary implications of recent and on-going interspecific hybridization.     

Mitochondrial DNA variability of the pipefish Syngnathus abaster

This study provides data on the genetic structuring of the pipefish Syngnathus abaster in the western Mediterranean and Adriatic Seas. A total of 109 specimens were collected in brackish‐water biotopes. The control region and three other regions of the mitochondrial genome were analysed. The most relevant result was the high genetic structuring found by Bayesian inference (BI), maximum likelihood (ML) and network analyses, which were consistent in showing three well‐separated clusters of S. abaster populations. Furthermore, BI and ML did not support the monophyly of the taxon S. abaster. These results suggest the occurrence of a species complex in the study area, whose differentiation may have occurred since the Pleistocene. The results also show a very high genetic variability at the inter‐population level, with no shared haplotypes among sites. Evolutionary forces due to the fragmented nature of the brackish‐water habitats may account for the high genetic divergence found among the groups and populations. Finally, although dispersal by rafting over long distances may occasionally occur, this study suggests linear stepping‐stone model of colonization to be most likely. The complexity of the results obtained suggests that further studies are needed to elucidate the phylogeny of S. abaster.     

Systematic relationships of sympatric pipefishes (Syngnathus spp.): A mismatch between morphological and molecular variation

Analyses of mitochondrial DNA and morphological variation were performed on specimens of all five currently recognised Syngnathus pipefish species from the eastern Pacific Ocean with type localities currently considered to lie within the Californian marine biogeographic province: kelp pipefish Syngnathus californiensis, bay pipefish S. leptorhynchus, barred pipefish S. auliscus, barcheek pipefish S. exilis and chocolate pipefish S. euchrous. Results consistently differentiate S. auliscus from the other species and fail to distinguish all other specimens as distinct species, as indicated by extensive morphological overlap as well as incomplete lineage sorting and considerably low genetic divergence for 16s and coI genes(<1%). This study presents a taxonomic revision of eastern Pacific Syngnathus spp. and proposes the synonymy of S. leptorhynchus, S. euchrous and S. exilis, under the senior synonym, S. californiensis. There is still a need to study populations of Syngnathus spp. from north and south of the Californian province to assess whether these too are synonyms of the two-species recognised here.     

Genetic monogamy despite social promiscuity in the pot-bellied seahorse (Hippocampus abdominalis)

Sexual selection theory predicts a positive correlation between relative parental investment and mate choice. In syngnathid fishes (seahorses and pipefish), males brood offspring in specialized brooding structures. While female-female mating competition has been demonstrated in some pipefishes, all seahorses (genus Hippocampus) studied to date have been found to have conventional sex roles with greater male-male competition for access to mates despite possessing the most complex brood structures in the family. Although multiple mating is common in pipefish, seahorses are again exceptional, exhibiting strict genetic monogamy. Both demographic and behavioural explanations have been offered to explain the lack of multiple mating in seahorse species, but these hypotheses have not yet been explicitly addressed. We investigated mating systems and brood parentage of the pot-bellied seahorse, Hippocampus abdominalis, a temperate-water species that is socially promiscuous with conventional sex roles in laboratory populations. We observed promiscuous courtship behaviour and sex-role reversal in high density, female-biased field populations of H. abdominalis. We hypothesize that sex roles are plastic in H. abdominalis, depending on local population density and sex ratio. Despite promiscuous courtship behaviour, all assayed male seahorses were genetically monogamous in both laboratory and wild populations. Physiological limitations associated with embryo incubation may explain the absence of multiple mating in seahorses and may have played an important role in the development of the unique reproductive behaviour typical in these species.     

Microsatellite analysis of maternity and the mating system in the Gulf pipefish Syngnathus scovelli, a species with male pregnancy and sex-role reversal

Highly variable microsatellite loci were employed to study the mating system of the sexually dimorphic Gulf pipefish Syngnathus scovelli . In this species, like others in the family Syngnathidae, 'pregnant' males provide all parental care. Gulf pipefish were collected from one locale in the northern Gulf of Mexico, and internally carried broods of 40 pregnant males were analysed genetically. By comparing multilocus microsatellite fingerprints for the inferred mothers against expected genotypic distributions from the population sample, it was determined that: (i) only one male had received eggs from more than a single female; and (ii) on two separate occasions, two different males had received eggs from the same female. Given the high power to detect multiple matings by males, the first finding indicates that only rarely are individual males impregnated by multiple females during the course of a pregnancy. Conversely, given the lower power to detect multiple matings by females due to sampling constraints, the second finding suggests a high frequency of multiple successful matings by females. Thus, this population of Gulf pipefish displays a polyandrous genetic mating system. The relevance of these genetic findings is discussed with regard to the evolution of secondary sex traits in this species, and in other syngnathids.     

Sex-specific responses to fecundity selection in the broad-nosed pipefish

Fecundity selection, acting on traits enhancing reproductive output, is an important determinant of organismal body size. Due to a unique mode of reproduction, mating success and fecundity are positively correlated with body size in both sexes of male-pregnant Syngnathus pipefish. As male pipefish brood eggs on their tail and egg production in females occurs in their ovaries (located in the trunk region), fecundity selection is expected to affect both sexes in this species, and is predicted to act differently on body proportions of males and females during their development. Based on this hypothesis, we investigated sexual size dimorphism in body size allometry and vertebral numbers across populations of the widespread European pipefish Syngnathus typhle. Despite the absence of sex-specific differences in overall and region-specific vertebral counts, male and female pipefish differ significantly in the relative lengths of their trunk and tail regions, consistent with region-specific selection pressures in the two sexes. Male pipefish show significant growth allometry, with disproportionate growth in the brooding tail region relative to the trunk, resulting in increasingly skewed region-specific sexual size dimorphism with increasing body size, a pattern consistent across five study populations. Sex-specific differences in patterns of growth in S. typhle support the hypothesis that fecundity selection can contribute to the evolution of sexual size dimorphism.     

Character displacement in sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>: allozymes and behavior

Synopsis We analyzed variation in allozymes and mating preferences in 12 populations across much of the range of the sailfin molly, Poecilia latipinna. Sailfin mollies can be sympatric with its sexual parasite Amazon mollies, P. formosa. Amazon mollies must co-exist and mate with bisexual males of closely related species (including sailfin mollies) to induce embryogenesis but inheritance is strictly maternal. Where sailfin and Amazon mollies are sympatric there is evidence of reproductive character displacement as males show a significantly stronger mating preference for sailfin molly females over Amazon mollies compared to preferences of males from allopatric populations. From the allozyme data we found a moderate amount of genetic variation across all populations but this variation did not reveal significant partitioning between sympatric and allopatric populations. Additionally, we found no evidence for isolation by distance as genetic distance was not significantly correlated with geographic distance. While allozyme variation also did not significantly correlate with male mating preferences, there was a significant correlation between male mating preferences and geographic distance. This correlation between mating preferences and geographic distance may have arisen from coevolution with Amazon mollies resulting in reproductive character displacement. Taken together, the distribution of genetic and behavioral variation among sympatric and allopatric populations suggests that behavioral evolution has outpaced evolution at the allozyme loci we examined in P. latipinna.     

中华猕猴桃和美味猕猴桃自然居群遗传结构及其种间杂交渐渗

 利用9对SSR引物对中华猕猴桃（Actinidia chinensis）和美味猕猴桃（A. deliciosa）两近缘种的5个同域分布复合体和各自1个非同域分布居群进行了遗传多样性、居群遗传结构的分析以及种间杂交渐渗的探讨。结果表明：1）两物种共有等位基因比例高达81.13%,物种特有等位基因较少（中华猕猴桃：13.27%,美味猕猴桃：5.61%）,但共享等位基因表型频率在两近缘种间存在差异,而且与各同域复合体中两物种样本的交错程度或间距存在关联;2）两种猕猴桃均具有极高遗传多样性,美味猕猴桃的遗传多样性（H_o＝0 .749, PIC＝0.818）都略高于中华猕猴桃（H_o＝0.686,PIC＝0.799）;3）两猕 猴桃物种均具有较低的Nei’s居群遗传分化度,但AMOVA分析结果揭示种内异域居群间（F_ST=0.091 5）和同域复合体种间（F_ST=0.111 5）均存在一定程度的遗传分化;中华猕猴桃居群遗传分化（G_ST=0.086; F_ST=0.212 1）高于美味猕猴桃（G_ST= 0.080;F_ST=0.142 0）;4）同域分布复合体两物种间的遗传分化（G_ST=0.020）低于物种内异域居群间的遗传分化（中华猕猴桃：G_ST=0.086; 美味猕猴桃：G_ST=0.080）,同域复合体物种间的基因流（N_m＝7.89 -29.75）远远高于 同种异域居群间（中华猕猴桃：N_m =2.663; 美味猕猴桃：N_m=2.880）; 5）居群UPGMA聚类揭示在同一地域的居群优先聚类,个体聚类结果显示多数个体聚在各自居群组内,但各地理居群并不按地理距离的远近聚类,这与Mantel相关性检测所揭示的居群间遗传距离与地理距离没有显著性相关的结果一致。进一步分析表明两种猕猴桃的遗传多样性和居群遗传结构不仅受其广域分布、远交、晚期分化等生活史特性的影响,同时还与猕猴桃的染色体基数高 (x=29)、倍性复杂和种间杂交等因素密切相关,其中两种猕猴桃的共享祖先多态性和同域分布种间杂交基因渗透对两猕猴桃的居群遗传结构产生了重要影响。