Theresa M. Bert

AN EMPIRICAL TEST OF PREDICTIONS OF TWO COMPETING MODELS FOR THE MAINTENANCE AND FATE OF HYBRID ZONES: BOTH MODELS ARE SUPPORTED IN A HARD-CLAM HYBRID ZONE

Two models developed to discern the mode of selection in hybrid zones differ in some predictions. The tension‐zone model predicts that selection acts against hybrids and independently of the environment (endogenous selection) and that selection is invariant throughout the hybrid zone. The ecological selection‐gradient, or ecotone, model maintains that fitness of different genotypes varies in response to environmental variation (exogenous selection) and thus, that in a region of the zone, fitness of hybrids is at least equal to that of the parental species. Therefore, to assess the predominant mode of selection operating in a hybrid zone, it is fundamental to evaluate whether selection is acting specifically against hybrid individuals, that is, whether hybridity alone is the basis for deficiencies of hybrids, and to evaluate whether the relative fitness of hybrids versus that of pure species varies across the zone. In a hardclam (genus Mercenaria) hybrid zone located in a polyhaline lagoon in east‐central Florida, we used age‐specific and location‐specific analyses to determine that a hybrid deficit occurrs, that the deficit seems to be due to selection against hybrids, and that selection varies across the zone. Various measures of deviation from Hardy‐Weinberg equilibrium, linkage disequilibrium analyses, and shifts in allele frequencies at semidiagnostic loci support the idea that selection is strongest in the northern region of the lagoon, the zone of sympatry and hybridization. Southward, into the range of M. mercenaria (the numerically predominant species), the percentage of hybrids remains relatively high and selection against hybrids decreases. For some genetic linkage groups, selection for M. mercenaria alleles seems to be occurring, but selection seems to be acting principally against alleles characteristic of M. mercenaria and, to a lesser degree, for alleles characteristic of M. campechiensis (the rarer species). These findings and others from previous analyses we have done on this hybrid zone demonstrate that selection in the zone is complex, and that characteristics of both the tension‐zone and ecotone models are present. Supporting the tension‐zone model, selection against hybrids per se clearly occurs, but specific genotypes seem to be at a selective disadvantage, whereas others have a selective advantage, and selection operates differentially on the two parental species within the zone. Supporting the ecotone model, the strength of overall selection varies throughout the zone, and environmentally mediated selection in which each species and hybrids have an advantage in specific habitats occurs, but some selection against hybrids is invariant throughout the zone. Thus, the structure and genetic architecture of this hybrid zone appear to be products of a complicated interaction between both types of selective forces cited in the two competing models.

Population genetics of the blue crabCallinectes sapidus: modest population structuring in a background of high gene flow

To determine the genetic population structure of blue crabs (Callinectes sapidus Rathbun), electrophoretic allozyme analysis was performed on 750 individuals collected from 16 nearshore locations ranging from New York to Texas, USA. Twenty enzymes and non-enzymatic proteins coded by 31 presumptive loci were examined. Twenty-two loci were either monomorphic or polymorphic at less than theP95 level; alleles for these polymorphic loci were geographically dispersed. Allele frequencies for three of the remaining polymorphic loci were homogeneous over all populations, as were levels of polymorphism and heterozygosity. Phenograms generated by the UPGMA (unweighted pair-group method using arithmetic averages) and distance Wagner methods exhibited no geographic pattern in the clustering of populations. Estimates ofNem (effective number of migrants per generation between populations) indicated substantial gene flow, with aalues sufficiently high to infer panmixia between all blue crab populations from New York to Texas. However, despite this high level of gene flow, two striking patterns of geographic differentiation occurred: genetic patchiness and clinal variation. Allele frequencies atEST-2, GP-1, IDHP-2, DPEP-1, DPEP-2, andTPEP exhibited genetic patchiness on local and range-wide geographic scales, and allele frequencies atEST-2 varied temporally. Genetic patchiness in blue crabs is likely to be the result of the pre-settlement formation and subsequent settlement of genetically heterogeneous patches of larvae; allele frequencies of those larval patches may then be further modified through ontogeny by localized selection. In the Atlantic Ocean, a regional latitudinal cline ofEST-2 allele frequencies was superimposed on the range-wide genetic patchiness exhibited by that locus. This pattern against a background of high gene flow is highly likely to be maintained by selection. In estuaries along the Atlantic Ocean coast, a combination of low adult long-distance migration and a high retention rate of locally spawned larvae could serve to segregate populations and allow for the development of the geographic cline inEST-2. The Gulf of Mexico showed no apparent cline, perhaps due to long-distance migration of females in some regions of the Gulf, or to masking by genetic patchiness. These results emphasize the importance of both ecological and evolutionary time scales and structuring mechanisms in determining genetic population structure.

GENETIC DIVERSITY IN THE mtDNA CONTROL REGION AND POPULATION STRUCTURE IN THE PINK SHRIMP FARFANTEPENAEUS DUORARUM

Abstract The pink shrimp (Farfantepenaeus duorarum) is an ecologically and economically important penaeid species in the southeastern United States. The biological and economic importance of this species prompted us to conduct a molecular genetic assessment of pink shrimp genetic diversity and population genetic structure. We sequenced a 558-base portion of the mitochondrial DNA (mtDNA) control region from pink shrimp collected at five locations ranging from North Carolina to south Texas. Haplotype and nucleotide diversity values in pink shrimp are among the highest yet reported for decapods. In our samples, all haplotypes were unique, and the average nucleotide diversity was approximately 2%. Nucleotide divergence between haplotypes ranged 0.2–3.9%. A mismatch analysis produced a unimodal distribution of pairwise differences between haplotypes that is consistent with a historic rapid population expansion. A population expansion, high mutation rate in the control region, and large population size all probably contributed to the development of high genetic diversity. This level of genetic diversity suggests that the mtDNA control region might be useful as a genetic marker in peneaid shrimp used for aquaculture. We detected no population differentiation, suggesting that long-term dispersal and gene flow are high enough to maintain a genetically homogeneous population structure over the geographic distances used in this analysis. The continuous distribution and high numbers of pink shrimp around south Florida likely have contributed to the genetic homogeneity observed in pink shrimp inhabiting the Atlantic Ocean and Gulf of Mexico. These data provide a baseline from which the genetic diversity and population structure of pink shrimp, which are highly exploited and vulnerable to the varied impacts of regional shrimp aquaculture, can be appraised in the future.

High frequency of gonadal neoplasia in a hard clam (Mercenaria spp.) hybrid zone

The etiology of bivalve gonadal neoplasia has eluded invertebrate pathologists for over 20 yr. In a coastal Florida (USA) lagoon, where two species of hard clams (Mercenaria mercenaria and M. campechiensis) cooccur and hybridize, they exhibit a persistent, unusually high frequency of gonadal neoplasia. Hybridity, rather than environmental or other biological factors, appears to determine susceptibility, implicating a genetic mechanism in the etiology of the disease. However, the increase in frequency of occurrence of the disease in hybrids is not accompanied by an increase in severity beyond that experienced by pure-species genotypes, suggesting that only some components of the genetic mechanism are affected by hybridization. Differences between sexes in the overall and size-specific frequency of occurrence and in severity of the disease suggest that the genetic mechanism is associated with sex. The excessive susceptibility of hybrid genotypes to gonadal neoplasia results in reduced hybrid fitness and constitutes an unambiguous example of a cellular disease that acts as a barrier to gene flow between species. Moreover, because these species are of commercial importance, fishery practices that promote hybridization are common and, over time, may reduce the fitness of natural populations.

Habitat-specific growth of hard clams Mercenaria mercenaria (L.) from the Indian River, Florida

Abstract Microgeographic variation in shell growth rate of Mercenaria mercenaria (L.) among habitats within the Indian River, Florida, was studied to develop a baseline against which the significance of M. mercenaria shell growth rate differences observed over a larger geographic scale could be compared. Hard clams of the genus Mercenaria were collected from 525 randomly located stations within the Indian River during July and August, 1986. Clams ( n = 922) were returned to the laboratory and taxonomically identified using diagnostic protein loci. Clams identified as M. mercenaria ( n = 465) were isolated and their shell growth rate determined by internal shell growth-increment analysis. The annual pattern of internal shell growth-increment formation in genetically identified M. mercenaria from the Indian River was documented using specimens collected monthly from September 1987 through August 1988. The translucent (slow growth) increment was deposited during summer and fall, whereas the remainder of the year was characterized by formation of the opaque (rapid growth) increment. This pattern is similar to that described for other hard clam populations from the southeastern United States. To compare shell growth rates of M. mercenaria among habitats within the Indian River, we used the ω parameter of Gallucci and Quinn. The range of variation in ω among habitats within the Indian River was similar to that observed for hard clams in Narragansett Bay, Rhode Island, but less than that observed for hard clams from various sites throughout the state of Florida. Shell growth rates of M. mercenaria in the Indian River increased with decreasing depth, and shell growth rates were more rapid in areas characterized by the presence of submerged aquatic vegetation than in areas devoid of such vegetation. Results of this study indicate that comparisons of shell growth rates among hard clam populations over a wide geographic range must be tempered by a consideration of shell growth rate variation among habitats within each site.

Time-averaging, evolution, and morphologic variation

Abstract Many fossil assemblages are time-averaged, with multiple generations of organisms mixed into a single stratigraphic horizon. A time-averaged sample of a taxon should be more variable than a single-generation sample if enough morphologic change occurred during the interval of time-averaging. Time-averaging may also alter correlations between morphologic variables and obscure allometric relationships in an evolving population. To investigate these issues, we estimated the variability of six modern, single-generation samples of the bivalve Mercenaria campechiensis using Procrustes analysis and compared them with several time-averaged Pleistocene samples of M. campechiensis and M. permagna. Both the modern and the fossil samples ranged in variability, but these ranges were virtually identical. Morphology was quite stable over the hundreds to many thousands of years that passed as the assemblages accumulated, and the variabilities of the fossil samples could be used to estimate single-generation variability. At one fossil locality, the environment and paleocommunity changed partway through the collection interval; the morphology of Mercenaria appears stable above and below the transition but changes across it. This change is similar in magnitude to the differences between geographically separated modern populations, whereas temporal variation within single environmental settings is distinctly less than geographic variation. Analytical time-averaging (the mixing of fossils from different horizons) between paleocommunities increased variability slightly (but not significantly) above that found in living populations. While its constituent populations appear stable on millennial timescales, M. campechiensis has been evolutionarily static since at least the early to middle Pleistocene.

The effects of temperature and salinity on survival and development of early life stage Florida stone crabs Menippe mercenaria (Say)

Abstract The effects of temperature and salinity on survival and growth of early life stage Menippe mercenaria (Say) were tested by rearing larvae and small juveniles (⩽10 mm CW) in factorial temperature-salinity arrays (larvae: 20–35°C [5°C increments], 10–40ℵ [10t% increments]; juveniles: 5–35°C [5°C increments], 10–40ℵ [5ℵ increments]). The highest proportion of larvae survived in water of 30°C, 30ℵ; juvenile survival was 100% in water ranging from 15 to 25°C and from 25 to 40ℵ. The distribution of this species both latitudinally and seaward may be determined by temperature and salinity tolerance limits of early stages. Both temperature and salinity affected the survival of early zoeal stages (Z1–3). Salinity effects decreased during late zoeal stages (Z4–Z5) and then increased in the megalopal and juvenile stages. The decrease in the effect of salinity on survival at zoeal stage four suggests that osmoregulatory ability begins at this stage. The occurence of a supernumerary zoeal stage (Z6) was not influenced by temperature or salinity and did not affect survival. Both larval developmental rates and frequency of molting in juveniles accelerated with increasing temperature but were not influenced by salinity. The observed effects of temperature on molting would promote rapid growth when water temperatues are warm, but would delay this physiologically stressful event when temperatures are cool and marginal for survival. The lower optimal temperature for survival of juveniles is probably an adaptation brought about by timing of the major reproductive season.

Genetic Management of Hatchery-Based Stock Enhancement

Including genetic considerations in stock enhancement can reduce the probability that enhanced (admixed) populations will undergo damaging genetic alteration through the stock enhancement effort. Avoiding alterations in genetic diversity, decreases in fitness, and reductions in effective population size (Ne) of admixed populations and their wild-population components is important for the long-term sustainability of those populations. Maintaining the genetic diversity of admixed populations and their wild-population components first requires managing both the genetic variability (e.g., numbers of alleles) and the genetic composition (frequencies of alleles) in the broodstocks and the broods. These genetic diversity components should be maintained at levels appropriate for each stock enhancement program throughout all aspects of stock enhancement—from broodstock selection through the rearing and releasing the broods and then, after release of the broods, in the admixed populations and their wild-population components until the admixed populations attain genetic equilibrium. Using small numbers of broodstock individuals, unequal contributions of broodstock individuals to broods, and inbreeding in broodstocks are common causes of alterations in genetic diversity. These pitfalls should be avoided because they can reduce genetic variability, change genetic composition, and increase genetic load (accumulation of deleterious alleles), which decreases the fitness hatchery broods. In an admixed population, reduction in the fitness of any population component (hatchery, wild, or their progeny) decreases the overall fitness of the admixed population. Hatchery brood fitness can also be reduced through outbreeding, which also ultimately decreases the fitness of the admixed population. Decreases in fitness of admixed populations or of any population components can extend over generations, particularly if stocking is repeated over multiple generations. The Ne of a population is directly related to losses in fitness due to inbreeding and reductions in genetic diversity. The smaller the Ne value of a population, the greater the chance that relatives will mate and that alleles (particularly rare alleles) will be lost over generations. Genetic monitoring programs for specific stock enhancement efforts Bert: Ecological and Genetic Implications ch08 Final Proof page 123 9.6.2007 11:40am Compositor Name: PDjeapradaban Theresa M. Bert (ed.), Ecological and Genetic Implications of Aquaculture Activities, 123–174.

Contemporaneous deposition of annual growth bands in Mercenaria mercenaria (Linnaeus), Mercenaria campechiensis (Gmelin), and their natural hybrid forms

Abstract We analyzed the periodicity of annual band deposition in the shells of each of three hard clam genotype classes ( Mercenaria mercenaria (Linnaeus), Mercenaria campechiensis (Gmelin), and hybrid forms of the two species) collected from the Indian River lagoon, Florida. Terminal growth bands from each of 396 genetically identified specimens were studied, using the translucent–opaque staging method, to determine the seasonality of rapid vs. slow shell growth. The pattern of terminal growth-band formation that we describe for M. mercenaria from the Indian River is similar to patterns observed for conspecifics collected from Georgia and North Carolina; the translucent growth band (indicative of relatively slow shell growth) is formed during summer and fall, and the opaque growth band (indicative of relatively rapid shell growth) is formed during winter and spring. During summer and fall, growth-band formation in M. campechiensis and hybrid forms is similar to that observed for M . mercenaria . In contrast, during winter and spring subtle but significant deviations from the M . mercenaria growth pattern were noted. For M . campechiensis , relatively equal proportions of the translucent and opaque growth stages were recorded among specimens collected during both the winter and spring seasons. Hybrid forms exhibited a seasonal pattern of terminal growth-band formation reflecting characteristics of each of the two parental species. During winter, translucent and opaque terminal growth bands were equally represented among hybrids (as in M . campechiensis ), whereas in spring significantly more hybrids were in the opaque growth stage (as in M . mercenaria ). The consistency that we describe among genotype classes in the seasonal pattern of growth-band formation suggests that previously described geographic differences in those patterns are primarily mediated by environmental factors rather than by inherent genotypic differences among populations. Nevertheless, subtle differences in the proportions of opaque vs. translucent growth bands observed between species during the rapid-growth stage and the intermediate nature of the hybrid growth pattern at that time suggest that some genetically mediated divergence in the seasonality of growth has occurred during the evolution of these two species.

CHARACTER DISCRIMINATORY POWER, CHARACTER-SET CONGRUENCE, AND THE CLASSIFICATION OF INDIVIDUALS FROM HYBRID ZONES: AN EXAMPLE USING STONE CRABS (MENIPPE)

Many investigators categorize individuals from hybrid zones to facilitate comparisons among genotypic classes (e.g., parental, F1, backcross) for comparative studies in which components of fitness or geographic variation are being analyzed. Frequently, multiple character sets representing genetically independent traits are used to classify these individuals and various methodologies are employed to combine the classifications obtained from the different character sets. We adapted the principles of total evidence and taxonomic congruence (two formalized approaches used by systematists in formulating phylogenetic hypotheses) to address the problem of discriminating hybridizing species and classifying individuals from hybrid zones. As our model, we used two morphological (coloration and morphometric) and two molecular (allozyme and mitochondrial DNA restriction‐fragment‐length polymorphism) character sets that differentiate two stone crab species (Menippe adina and M. mercenaria). Using principal‐components analysis, we determined that combining character sets and eliminating characters or character sets that did not have large eigenvector coefficients for the principal component that best separated the two species yielded the highest level of discrimination between species and allowed us to classify a broad range of morpho‐genotypes as hybrids. For the stone crabs, three diagnostic allozyme loci and five diagnostic coloration characters best separated the species. The two character sets were not completely congruent, but they agreed in their classification of 50% of the individuals from the hybrid zone and rarely strongly disagreed in their classifications. Classification discrepancies between the two character sets probably represent variation between traits in interspecific gene flow rather than intraspecific, ecologically mediated variation. Our results support the assertions of previous investigators who espoused the benefits associated with using multiple character sets to classify individuals from hybrid zones and demonstrate that, if character sets are reasonably congruent and numerically balanced, combining diagnostic characters from multiple character sets (a total‐evidence approach) can enhance discriminatory power between species and facilitate the assignment of hybrid‐zone individuals to genotypic classes. On the contrary, classifying hybrid‐zone individuals using character sets separately (a taxonomic‐congruence approach) provides the opportunity to compare levels of introgression between species and to assess reasons for discordance among the data sets.