INCIPIENT REPRODUCTIVE ISOLATION BETWEEN TWO SYMPATRIC MORPHS OF THE INTERTIDAL SNAIL LITTORINA SAXATILIS

The study of speciation in recent populations is essentially a study of the evolution of reproductive isolation mechanisms between sub-groups of a species. Prezygotic isolation can be of central importance to models of speciation, either being a consequence of reinforcement of assortative mating in hybrid zones, or a pleiotropic effect of morphological or behavioral adaptation to different environments. To suggest speciation by reinforcement between incipient species one must at least know that gene flow occurs, or have recently occurred, and that assortative mating has been established in the hybrid zone. In Galician populations of the marine snail Littorina saxatilis, two main morphs appear on the same shores, one on the upper-shore barnacle belt and the other in the lower-shore mussel belt. The two morphs overlap in distribution in the midshore where hybrids are found together with pure forms. Allozyme variation indicates that the two parental morphs share a common gene pool, although within shores, gene flow between morphs is less than gene flow within morphs. In this study, we observed mating behavior in the field, and we found that mating was not random in midshore sites, with a deficiency of heterotypic pairs. Habitat selection, assortative mating, and possibly sexual selection among females contributed to the partial reproductive isolation between the pure morphs. Sizes of mates were often positively correlated, in particular, in the upper shore, indicating size-assortative mating too. However, this seemed to be a consequence of nonrandom microdistributions of snails of different sizes. Because we also argue that the hybrid zone is of primary rather than secondary origin, this seems to be an example of sympatric reproductive isolation, either established by means of reinforcement or as a by-product to divergent selection acting on other characters.     

MORPHOLOGICAL DIVERGENCE AND EVIDENCE FOR REPRODUCTIVE ISOLATION IN LITTORINA SAXATILIS(OLIVI) IN NORTHEAST ENGLAND

Current work at Old Peak (Ravenscar) on the northeast coastof England indicates that boulder-dwelling Lutonna saxatilis(Olvi) comprise at least two distinct forms, inhabiting high-and mid-shore boulders Using multivariate morphometrics we showthat these forms are significantly different in shape. Rareintermediates between these two distinct forms were occasionallyfound, usually with the low shore animals The two forms showdifferent reproductive strategies. High shore females (L. saxatilisH) mature at a smaller size and produce a smaller number oflarge eggs giving rise to large crawlaways The mid-shore form(L. saxatilis M) matures at a larger size and produces a largenumber of small eggs which give rise to small crawlaways. Anovel observation is that intermediate females (L. saxatilisI) contain eggs of both sizes, giving rise to a bimodal distributionof egg sizes within the brood pouch. The usual incidence ofabortion (observed in H and M forms) is Very low (<1%) However,in the intermediates the large eggs showed a high incidenceof abortion (about 60%) and the incidence of abortion in thesmall eggs was also increased (about 30%) Thus there appearsto be a partial reproductive barrier between the two forms,presumably resulting in reduced fitness due in the intermediatesto reproductive dysfunction. These findings extend to othershores in the region, over total distance of about 66 km. Thefacts are suggestive of a hitherto unrecognised reproductivebarrier with L. saxatilis, the nature and strength of whichis under investigation ^*Present address: University College Scarborough. Filey Road.Scarborough Yo11 3AZ. (Received 12 June 1995; accepted 15 September 1995)     

THE MAINTENANCE OF A CLINE IN THE MARINE SNAIL LITTORINA SAXATILIS: THE ROLE OF HOME SITE ADVANTAGE AND HYBRID FITNESS

Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain steep genetic clines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the cline (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the cline may be used to distinguish between these models to assess how the cline is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct-developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1–38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39-52% of the native ecotype on upper shores, but only 2-8% for the lower-shore ecotype. In contrast, only 4-8% of the upper-shore ecotype but 53% of large (> 6 mm) and 8% of small (3-6 mm) native lower-shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs, hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross-shore selection gradients, thus supporting the selection-gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.     

THE GENETIC ARCHITECTURE OF REPRODUCTIVE ISOLATION IN LOUISIANA IRISES: HYBRID FITNESS IN NATURE

Negative epistasis in hybrid genomes commonly results in postzygotic isolation between divergent lineages. However, some genomic regions may be selectively neutral or adaptive in hybrids and thus may potentially cross species barriers. We examined postzygotic isolation between ecologically similar species of Louisiana Iris: Iris brevicaulis and I. fulva to determine the potential for adaptive introgression in nature. Line-cross analyses allowed us a general overview of the gene action responsible for fitness-related traits. We then used a QTL mapping approach to detect genomic regions responsible for variation in these traits. Although hybrid classes suffered reduced fitness for many traits, hybrid means were equivalent to at least one of the parental species in overall estimates of maternal and paternal fitness during the two years of the field study. The genetic architecture underlying the fitness-related traits varied across field site and year of the study, thus emphasizing the importance of the environment in determining the degree of postzygotic isolation and potential for introgression across natural hybrid zones.     

ANNUAL AND LUNAR REPRODUCTIVE CYCLES IN LITTORINA SAXATILIS (OLIVI) AND DIFFERENCES BETWEEN BREEDING IN THE MARINE FIRTH OF FORTH AND THE FORTH ESTUARY

Gonads were most commonly reduced in October and fullest inMar-Jun with more pronounced seasonal differences in the estuarythan in the firth. Brood-pouches accordingly were fullest inspring-early summer and emptiest in autumn-early winter. Eggsand early embryos predominated in brood-pouches of early springbut were few in Aug-Oct whereas well-developed juveniles werefew in early spring, increased in spring-summer and came topredominate in Jun-Nov. Brood-pouch loads varied with size offemale but, for a standard 10 mm female, were significantlylower in the 2 estuarine sites than in the firth. Changes inmean brood-pouch count varied only 2.5-fold during the yearin the firth but 4.10-fold in the estuary. In vitro development of eggs to shelled juveniles with the velumresorbed took some 68 d at 10°C and 32%. Eggs and late brood-pouchjuveniles were significantly heavier in the estuary than inthe firth (dry weight, ash-free dry weight (AFDW), ash weight).AFDW changed little during development while ash weight (mostlyshell) increased 3.5-4.4-fold. In a laboratory tide tank, femalesreleased 60 d means of <1.3 juveniles d^–1; more inmid- and late year than in Jan-Mar, more from upshore firthfemales than from low shore or estuary females, more when heldat high tide-tank levels than at lower levels, and generallyat rates comparable to release in containers held at the siteof origin. Egg production in laboratory females did not keeppace with release of young and, at the end of trials, brood-pouchcounts were mostly lower than in contemporary free winkles.Females that had released most (e.g. at high tank levels) containedleast brood-pouch embryos and young subsequently. Release of young in the tide tank was markedly faster at timesnear new moon (often reaching or exceeding 2 female^–1d^–1) than near full moon (often <1 female^–1 d^–1).This lunar fluctuation was clearest in fast-releasing sets heldat high- or mid-tide tank levels. It was not observed in Jan-Mar(when release from most females was slow) nor in females atlow tank levels. (Received 3 September 1990; accepted 8 November 1990)     

GENETIC EVIDENCE FOR REPRODUCTIVE ISOLATION AND MULTIPLE ORIGINS OF SYMPATRIC TROPHIC ECOTYPES OF WHITEFISH (COREGONUS)

We assessed variation in mitochondrial DNA (mtDNA) by restriction fragment length polymorphism (RFLP) analysis and in nuclear genes by allozyme analysis among sympatric pairs of limnetic and benthic ecotypes of whitefish (Coregonus) coexisting in three lakes of southern Yukon to address three evolutionary questions regarding their origins. Are sympatric low and high gill-raker count ecotypes genetically differentiated? Are they issued from monophyletic or polyphyletic evolutionary events? If they are polyphyletic in origins, did they originate from multiple allopatric speciation events or intralacustrine radiation? Our results corroborated previous genetic and ecological studies of these ecotypes, indicating that they represent genetically distinct reproductive units, and therefore refuting the hypothesis of phenotypic polymorphism within a single population. However, the amount of gene flow between ecotypes varied among lakes, correlating with the extent of morphological differentiation and the potential for premating reproductive isolation. The results indicated a polyphyletic origin of ecotypes whereby each of them have been expressed independently more than once. In the two lakes of Squanga Creek drainage, the existence of sympatric pairs was best explained by the secondary contact of two monophyletic whitefish groups that evolved in allopatry during the last glaciation events. In Dezadeash L. of Alsek R. drainage, our results could not verify either sympatric or allopatric (or microallopatric) origin of ecotypes. Regardless of the mode of speciation involved in their origins, these sympatric whitefish populations provided further evidence that Pleistocene glaciation events created conditions favoring rapid divergence and phenotypic differentiation among northern freshwater fishes.     

UNIFICATION OF METHODS FOR ESTIMATING THE STRENGTH OF REPRODUCTIVE ISOLATION

Understanding the evolution of reproductive isolation is tantamount to describing the origin of species. Therefore, a primary goal in evolutionary biology is to identify which reproductive barriers are most important to the process. To achieve this goal, the strength of multiple forms of isolation must be compared in an equivalent manner. However, a diversity of methods has been used to estimate barrier strength, falling into several mathematically distinct categories. This study provides a unified method for calculating isolation that relates the amount of gene flow experienced by taxa to random expectations in a simple linear framework. This approach has three distinct advantages over previous methods: (1) it is directly related to gene flow, (2) it is symmetrical, such that measures in both the positive and negative range are comparable, and (3) it is equivalent between broad categories of reproductive isolation, allowing for appropriate comparisons. This linear formulation can be adjusted for use in all forms of isolation, and can accommodate cases in which null expectations for con‐ and heterospecific gene flow differ. Additionally, this framework can be used to calculate total reproductive isolation and the relative contributions of individual barriers.     

CONTRIBUTION OF MULTIPLE ISOLATING BARRIERS TO REPRODUCTIVE ISOLATION BETWEEN A PAIR OF PHYTOPHAGOUS LADYBIRD BEETLES

Reproductive isolation between species may often be attained by multiple isolating barriers, but the components are rarely studied in animal taxa. To elucidate the nature of multiple isolating barriers, we quantified the strength of three premating barriers, including ecologically based ones (seasonal, habitat, and sexual), two postmating–prehatching barriers (reduced egg hatchability and conspecific sperm precedence [CSP]), and one posthatching barrier, including four components of F₁ hybrid reduced fitness, between two phytophagous ladybird beetles, Henosepilachna vigintioctomaculata and H. pustulosa . We detected five positive barriers (habitat isolation, sexual isolation, reduced egg hatchability, CSP, and reduced egg hatchability in backcrosses of F₁ hybrids). None of these barriers entirely prevents gene exchange when it acts alone, but jointly they generate nearly complete reproductive isolation even between sympatric populations. Host fidelity contributed most strongly to reproductive isolation by reducing interspecific hybridization through several important types of ecological isolation, including microspatial, habitat, and seasonal isolation. The existence of multiple isolating barriers likely helps keep reproductive isolation stable and robust, by complementing changes in the strength of leaky barriers. This complementarity of multiple isolating barriers yields the concept of robustness of reproductive isolation, which is important when considering the long-term maintenance of species boundaries in coexisting species pairs.     

DIVERGENCE OF MEIOTIC DRIVE-SUPPRESSION SYSTEMS AS AN EXPLANATION FOR SEX-BIASED HYBRID STERILITY AND INVIABILITY

Two empirical generalizations about speciation remain unexplained: the tendency of the heterogametic sex to be sterile or inviable in F₁ hybrids (Haldane's rule), and the tendency of the X chromosome to harbor the genetic elements that cause this sex bias in hybrid fitness. I suggest that divergence of meiotic drive systems on the sex chromosomes can explain these observations. The theory follows from two simple facts. First, sex chromosomes are particularly susceptible to the forces of meiotic drive. Second, divergence of meiotic drive systems can cause hybrid sterility and in viability. The main objection to the theory is that meiotic drive is apparently rare, whereas the observed pattern of hybrid fitness is widespread. I answer this objection by showing that divergence of meiotic drive systems can explain the two generalizations even if large departures from Mendelian segregation are rarely observed.     

COMPONENTS OF REPRODUCTIVE ISOLATION BETWEEN NORTH AMERICAN PHEROMONE STRAINS OF THE EUROPEAN CORN BORER

Of 12 potential reproductive isolating barriers between closely related Z‐ and E‐pheromone strains of the European corn borer moth (Ostrinia nubilalis), seven significantly reduced gene flow but none were complete, suggesting that speciation in this lineage is a gradual process in which multiple barriers of intermediate strength accumulate. Estimation of the cumulative effect of all barriers resulted in nearly complete isolation (>99%), but geographic variation in seasonal isolation allowed as much as ～10% gene flow. With the strongest barriers arising from mate‐selection behavior or ecologically relevant traits, sexual and natural selection are the most likely evolutionary processes driving population divergence. A recent multilocus genealogical study corroborates the roles of selection and gene flow ( Dopman et al. 2005 ), because introgression is supported at all loci besides Tpi, a sex‐linked gene. Tpi reveals strains as exclusive groups, possesses signatures of selection, and is tightly linked to a QTL that contributes to seasonal isolation. With more than 98% of total cumulative isolation consisting of prezygotic barriers, Z and E strains of ECB join a growing list of taxa in which species boundaries are primarily maintained by the prevention of hybridization, possibly because premating barriers evolve during early stages of population divergence.     

WOLBACHIA AND THE EVOLUTION OF REPRODUCTIVE ISOLATION BETWEEN DROSOPHILA RECENS AND DROSOPHILA SUBQUINARIA

Endosymbiotic bacteria of the genus Wolbachia are widespread among insects and in many cases cause cytoplasmic incompatibility in crosses between infected males and uninfected females. Such findings have been used to argue that Wolbachia have played an important role in insect speciation. Theoretical models, however, indicate that Wolbachia alone are unlikely to lead to stable reproductive isolation between two formerly conspecific populations. Here we analyze the components of reproductive isolation between Drosophila recens, which is infected with Wolbachia, and its uninfected sister species Drosophila subquinaria. Laboratory pairings demonstrated that gene flow via matings between D. recens females and D. subquinaria males is hindered by behavioral isolation. Matings readily occurred in the reciprocal cross (D. quinaria females × D. recens males), but very few viable progeny were produced. The production of viable hybrids via this route was restored by antibiotic curing of D. recens of their Wolbachia symbionts, indicating that hybrid offspring production is greatly reduced by cytoplasmic incompatibility in the crosses involving infected D. recens males. Thus, behavioral isolation and Wolbachia-induced cytoplasmic incompatibility act as complementary asymmetrical isolating mechanisms between these two species. In accordance with Haldane's rule, hybrid females were fertile, whereas hybrid males invariably were sterile. Levels of mtDNA variation in D. recens are much lower than in either D. subquinaria or D. falleni, neither of which is infected with Wolbachia. The low haplotype diversity in D. recens is likely due to an mtDNA sweep associated with the spread of Wolbachia. Nevertheless, the existence of several mtDNA haplotypes in this species indicates that Wolbachia have been present as a potential isolating mechanism for a substantial period of evolutionary time. Finally, we argue that although Wolbachia by themselves are unlikely to bring about speciation, they can increase the rate of speciation in insects.     

AN INTERACTION BETWEEN ENVIRONMENTAL TEMPERATURE AND GENETIC VARIATION FOR BODY SIZE FOR THE FITNESS OF ADULT FEMALE DROSOPHILA MELANOGASTER

Abstract. — Drosophila and other ectotherms show geographic genetic variation in body size, with larger individuals at higher latitudes and altitudes. Temperature is implicated as an important selective agent because long-term laboratory culture of Drosophila leads to the evolution of larger body size at lower temperatures. In this paper, we tested the hypothesis that, in Drosophila melanogaster, larger size is favored at lower temperatures in part because of selection on adult females. We used replicated lines of D. melanogaster artificially selected for increased and decreased wing area with constant cell area. The resulting size differences between the selected lines were due solely to differences in cell number, and thereby were similar to the cellular basis of clinal variation in body size in nature. We examined life-history traits of adult females at 18 and 25°C. Rearing for two generations at the two temperatures did not affect the extent of the size differences between lines from the different selection regimes. There was a strong interaction between temperature and size selection for both survival and lifetime reproductive success, with larger females living significantly longer and producing more offspring over their lifetime only when reared and tested in the colder environment. There was also an increase in average daily progeny production in large-line females relative to the control and small lines again, only in the colder environment. Thus, the females from the large selection lines were relatively fitter at the colder temperature. At both experimental temperatures, especially the lower one, the small- line females rescheduled their progeny production to later ages. Larger body size may have evolved at higher latitudes and altitudes because of the advantages to the adult female of being larger at lower temperatures.     

AN ISOLATION UNIT FOR SUPPLYING RADIOISOTOPES TO SPECIFIC SEGMENTS OF AN INTACT ROOT

A device useful for supplying radioactive ions to specific segments of intact roots is described. A small volume of aerated, radioactive culture solution may be circulated around an isolated segment of a root, while the remainder of that root and the other roots of the plant are exposed to non-radioactive culture solution. Any root which does not have lateral roots extending beyond the epidermis can be inserted into the device without handling the root itself. No sealants are required to achieve a leak-proof seal above and below the root segment. The use of flexible rubber membranes to isolate root segments causes no detectable damage to root cells where the membranes contact the root epidermis.     

DEVELOPMENT OF AN IMPROVED MEDIUM FOR THE ISOLATION OF LIVER MITOCHONDRIA

In view of the unsatisfactory appearance, under the electron microscope, of liver mitochondria isolated in isotonic sucrose medium, alternative media have been examined. It was found to be advantageous to replace sucrose by raffinose, and to add levan or, preferably, dextran, together with heparin in suitable concentration. With the optimal medium, the constituents of which are raffinose, versene (optional), dextran of high molecular weight, heparin, and AMP (optional), most of the mitochondria in the osmium-fixed pellet are apparently intact, and show the membranes characteristic of mitochondria as seen in cell sections. The optimal medium has no adverse effect on the activity of the several tissue enzymes which have been studied, except that Mg⁺⁺-activated ATPase is partially inhibited if the medium is present in high concentration in the assay system. Mitochondrial fractions isolated in the new medium have, in common with sucrose fractions, appreciable "free" ATPase activity, this activity being evidently a poor criterion of mitochondrial integrity. Use of the new medium does not decrease the proportion of cytoplasmic ATPase which fails to sediment with the mitochondria, but does give a mitochondrial fraction low in RNA and in acid phosphatase activity and little contaminated with microsomal material. Particles tentatively identified as "lysosomes" have been seen in certain sections.     

DISRUPTIVE SELECTION ON HABITAT PREFERENCE AND THE EVOLUTION OF REPRODUCTIVE ISOLATION: AN EXPLORATORY EXPERIMENT

A. pair of replicate experiments was carried out to examine the role of habitat specialization in the process of sympatric speciation. The rationale of the experimental design was that disruptive selection for habitat preference can facilitate the process of sympatric speciation by reducing an antagonistic interaction between the processes of recombination and selection. Replicate populations of Drosophila melanogaster were subjected to disruptive selection for habitat preference and simultaneous selection for positive assortative mating, for a period of 46 generations. The experimental protocol simulated the introduction of a frugivorous fly population into a new environment that contained a mosaic of eight equally abundant habitats. Only two habitats were suitable to the flies, and these required opposite patterns of geotaxis, phototaxis, chemotaxis, and developmental time to be successfully located. At the beginning of each generation, flies were placed as pupae into the middle of a maze containing eight discrete habitats. To produce disruptive selection on habitat preference, only those flies that chose one or the other of two selected habitats were permitted to contribute gametes to the next generation. Flies from each of the selected habitats were cultured separately to simulate independent carrying capacities within each habitat. Because flies rarely mated until they assorted themselves into the available habitats, genes affecting habitat preference pleiotropically produced assortative mating between flies with similar habitat preference. A genetic marker was used to continuously monitor the level of potential gene flow between the flies derived from the two selected habitats. During the course of the experiment, there was a gradual increase in the philopatry of the flies, indicating the development of habitat specialization. The evolution of habitat specialization resulted in substantial reproductive isolation between the subpopulations utilizing each habitat resource.     

MITOCHONDRIAL–NUCLEAR EPISTASIS AFFECTS FITNESS WITHIN SPECIES BUT DOES NOT CONTRIBUTE TO FIXED INCOMPATIBILITIES BETWEEN SPECIES OF DROSOPHILA

Efficient mitochondrial function requires physical interactions between the proteins encoded by the mitochondrial and nuclear genomes. Coevolution between these genomes may result in the accumulation of incompatibilities between divergent lineages. We test whether mitochondrial–nuclear incompatibilities have accumulated within the Drosophila melanogaster species subgroup by combining divergent mitochondrial and nuclear lineages and quantifying the effects on relative fitness. Precise placement of nine mtDNAs from D. melanogaster, D. simulans, and D. mauritiana into two D. melanogaster nuclear genetic backgrounds reveals significant mitochondrial–nuclear epistasis affecting fitness in females. Combining the mitochondrial genomes with three different D. melanogaster X chromosomes reveals significant epistasis for male fitness between X‐linked and mitochondrial variation. However, we find no evidence that the more than 500 fixed differences between the mitochondrial genomes of D. melanogaster and the D. simulans species complex are incompatible with the D. melanogaster nuclear genome. Rather, the interactions of largest effect occur between mitochondrial and nuclear polymorphisms that segregate within species of the D. melanogaster species subgroup. We propose that a low mitochondrial substitution rate, resulting from a low mutation rate and/or efficient purifying selection, precludes the accumulation of mitochondrial–nuclear incompatibilities among these Drosophila species.     

THE CONTRIBUTION OF NEW MUTATIONS TO GENOTYPE-ENVIRONMENT INTERACTION FOR FITNESS IN DROSOPHILA MELANOGASTER

Many studies have documented the existence of genotype-environment interaction (GEI) for traits closely related to fitness in natural populations. A type of GEI that is commonly observed is changes in the fitness ranking of genetic groups (families, clones, or inbred lines) in different environments. We refer to such changes in ranking as crossing of reaction norms for fitness. A common interpretation of crossing of reaction norms for fitness is that selection favors different alleles in the different environments (i.e., that “trade-offs” exist). If this is the case, selection could maintain genetic variation, and even lead to reproductive isolation between subpopulations using different environments. Even if the same alleles are favored in every environment, however, deleterious mutations that vary in the magnitude of their effect depending on environment could cause reaction norms for fitness to cross. If deleterious mutations with environment-dependent effects are responsible for maintaining much of the variation leading to crossing of reaction norms for fitness in natural populations, it should be possible to observe crossing of reaction norms for fitness among otherwise genetically identical lines bearing newly arisen spontaneous mutations. We examined the contribution of new mutations to GEI for fitness in Drosophila melanogaster. Eighteen lines were derived from a common, highly inbred base stock, and maintained at a population size of 10 pairs for over 200 generations, to allow them to accumulate spontaneous mutations. Because of the small population size of the lines, selection against mildly deleterious mutations should have been relatively ineffective. The lines were tested for productivity (number of surviving adult progeny from a standard number of parents) in five different environmental treatments, comprising different food media, temperatures, and levels of competition. The lines showed highly significant GEI for productivity, owing largely to considerable changes in ranking in the different environments. We conclude that mutations that are deleterious on average, but whose quantitative effects depend on environment, could be responsible for maintaining much of the variation leading to crossing of reaction norms for fitness that has been observed in samples of D. melanogaster from the wild.     

SIGNIFICANT ROLE FOR HISTORICAL EFFECTS IN THE EVOLUTION OF REPRODUCTIVE ISOLATION: EVIDENCE FROM PATTERNS OF INTROGRESSION BETWEEN THE CYPRINID FISHES,LUXILUS CORNUTUS AND LUXILUS CHRYSOCEPHALUS

Samples of Luxilus cornutus, Luxilus chrysocephalus, and their hybrids were collected along hypothesized routes of dispersal from Pleistocene refugia to examine the significance of geographic variation in patterns of introgression between these species. Patterns of allozyme and mitochondrial DNA (mtDNA) variation were generally consistent with those from previous studies. Tests of Hardy-Weinberg equilibrium revealed significant deficiencies of heterozygotes in all samples, indicating some form of reproductive isolation. Mitochondrial DNAs of each species were not equally represented in F₁ hybrids; however, this bias was eliminated when the two largest samples were excluded from the analysis. Backcross hybrids exhibited biased mtDNA introgression, as samples from Lake Erie (eastern) and Lake Michigan (western) drainages showed significant excesses of mtDNAs from L. chrysocephalus and L. cornutus, respectively, relative to frequencies of diagnostic allozyme markers. The extent and direction of allozyme and mtDNA introgression was quantified by calculating isolation index values from morphologically “pure” individuals of each species from each locality. Analysis of variance of these measures identified limited introgression of allozyme variants with no geographic pattern, but significant differences in direction of mtDNA introgression between drainages (i.e., postglacial dispersal route). Association between patterns of mtDNA introgression and dispersal route across the latitudinal width of the contact zone is best explained by genetic divergence during past isolation of ancestral populations from these drainages. These results identify a significant role for historical effects in the evolution of reproductive isolation and the process of speciation.