Hybridization and phylogeography of the Mozambique tilapia <Emphasis Type="Italic">Oreochromis mossambicus</Emphasis> in southern Africa evidenced by mitochondrial and microsatellite DNA genotyping

Many Oreochromis species utilized in aquaculture were extensively introduced outside their native range in Africa. Given their recent evolutionary radiation, these species hybridize easily, posing a threat to the integrity of local adaptation. The objective of this work was to study the genetic diversity of the Mozambique tilapia (Oreochromis mossambicus) in its native range, southern Africa, and provide a method for identifying hybrids with genetic markers. We genotyped the mitochondrial DNA (mtDNA) control region (385 bp) of wild and farmed O. mossambicus, wild and farmed O. niloticus and morphologic wild hybrids. These data were complemented with published sequences of parapatric and sympatric Oreochromis taxa. Phylogeographic analysis showed the presence of two O. mossambicus lineages, the southernmost representing a recent Holocene radiation. Hybridization of O. mossambicus was indicated by the presence of O. niloticus and O. mortimeri–andersonii mtDNA specimens in the Limpopo basin and of O. karongae mtDNA in specimens from Malawi. We also genotyped seven suspected hybrid individuals from the Limpopo River, and 137 wild and farmed Mozambique and Nile tilapia samples with five microsatellite markers. Factorial Component Analysis, Bayesian clustering and assignment analyses consistently delineated an O. mossambicus and an O. niloticus group, with the putative hybrids positioned in between. Different levels of hybridization were detected by the Bayesian assignment. The complex nature of hybridization and introgression between cichlid species raises major concerns for the long-term integrity of Mozambique tilapia.     

COMPARISONS OF THE VOCALIZATIONS AND SOCIAL BEHAVIOUR OF SOUTHERN AFRICAN PYCNONOTUS BULBULS

Lloyd, P., Hulley, P.E. & Craig, A.J.F.K. 1996. Comparisons of the vocalizations and social behaviour of southern African Pycnonotus bulbuls. Ostrich 67: 118–125.

Vocalizations and associated behaviour of three Pycnonotus species are described, based on field observations and tape recordings from which sonagrams were produced. These species, which are locally sym-patric and hybridize, have similar vocalizations and displays; differences are most apparent in their contact calls and songs. Quantitative analysis of the songs showed that P. barbatus and P. capensis are easily distinguished, whereas the song characteristics of P. nigricans overlap those of both the other species. Playback experiments with territorial male P. barbatus in an area of allopatry showed similar responses to songs of conspecifics and of P. nigricans, but almost no response to the song of P. capensis.     

REPRODUCTIVE ISOLATION AND INTROGRESSION BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (FAMILY CYPRINIDAE): COMPARISON OF MORPHOLOGY,ALLOZYMES, AND MITOCHONDRIAL DNA

Hybrid zones in fluvial fishes may be heterogeneous from drainage to drainage. The comparison of data from morphology, allozymes, and mitochondrial DNA (mtDNA) indicates variability in the causes and degree of restriction of gene flow between Notropis cornutus and Notropis chrysocephalus. Allozyme marker loci show frequency-dependent introgression; i.e., the rarer species, whichever it is at a particular locality, tends to exhibit a higher proportion of introgressed alleles. Unlike allozymes, introgression of mtDNA haplotypes varies geographically. In westward-flowing Michigan drainages, N. cornutus mtDNA haplotypes are more common in F₁ hybrids and backcrosses, independent of parental frequencies. In eastward-flowing Michigan drainages, N. chrysocephalus mtDNA is more common in F₁ hybrids and backcrosses; this pattern may be due to local ecological effects or frequency-dependent introgression. Morphological data alone are not sufficient to distinguish all classes of hybrids. The lack of concordance of morphological, allozymic, and mtDNA introgression patterns implies operation of one or two factors: 1) geographically variable patterns of selection against different hybrid and backcross combinations or 2) genetic differences between Michigan populations inhabiting eastward- and westward-flowing drainage systems accumulated during historical isolation.     

Song sharing by neighbourhood groups of territorial male Blackeyed Bulbuls

Lloyd, P., Hulley, P.E. & Craig, A.J.F.K. 1999. Song sharing by neighbourhood groups of territorial male Blackeyed Bulbuls. Ostrich 70 (3&4): 208–213

The territorial song of the Blackeyed Bulbul Pycnonotus barbatus is described for a population followed over two breeding seasons in suburban Grahamstown, South Africa. The Blackeyed Bulbul has a discrete and highly structured system of phrase sharing by neighbourhood groups of males. The most common repertoire size is five phrases (mean 5.5; range 2–9). In a temporally-stable song population, individual phrases are shared independently by discrete clusters of males. Neighbours show a strong tendency to share phrase types (but rarely whole repertoires), with overall song similarity decreasing rapidly with increasing distance between birds. This species is an ideal candidate for testing hypotheses concerning the adaptive value of song sharing by neighbourhood groups.     

Hybridisation and lack of prezygotic barriers between Phymata pennsylvanica and americana

1. The persistence of both geographical and reproductive boundaries between related species poses a fundamental puzzle in biology. Reproductive interactions between species can have a substantial impact on the maintenance of a boundary, potentially contributing to its collapse (e.g. via hybridisation) or facilitating reproductive isolation (e.g. via reinforcement). 2. The degree to which two parapatric insect species in the genus Phymata are reproductively isolated was evaluated and several mechanisms that could contribute to the maintenance of species boundaries were assessed. 3. Behavioural assays showed no indication of species‐assortative mating, nor any fecundity costs associated with heterospecific mating. Thus, there was no evidence of prezygotic mechanisms of reproductive isolation between the two species. 4.In laboratory crosses, it was found that the two species were indeed capable of producing viable F1 hybrids. Morphologically, these hybrids were phenotypically intermediate to the two parental species, and similar to the phenotypes seen in natural populations thought to occur in a hybrid zone. F1 hybrids did not show reduced viability, although there was some suggestion of ‘hybrid breakdown’, evident from the lower viability observed for progeny of ‘natural hybrids’. 5. Collectively, we show that despite genetically based morphological differences between species, P. americana and pennsylvanica can, and probably do hybridise. More studies are needed to understand the mechanisms that maintain the distinct phenotypes and geographical ranges of these species, despite the considerable potential for introgression.     

Molecular patterns of introgression in a classic hybrid zone between the Australian tree frogs,Litoria ewingii and L. paraewingi: evidence of a tension zone

Hybrid zones provide a rare opportunity to explore the processes involved in reproductive isolation and speciation. The southern hybrid zone between the southeastern Australian tree frogs Litoria ewingii and L. paraewingi has been comprehensively studied over the last 40 years, primarily using reproductive compatibility experiments and male advertisement calls. We used mitochondrial DNA (mtDNA) and eight nuclear microsatellite markers to characterize this hybrid zone along a historically studied transect and to test various dispersal‐dependent and dispersal‐independent hybrid zone models. The species are genetically distinct and the level of hybridization within the contact zone is low, with the majority of admixed individuals representing later‐generation hybrids. Based on previous experimental genetic compatibility studies, we predicted that hybrids with L. paraewingi mtDNA would be more frequent than hybrids with L. ewingii mtDNA. Surprisingly, a greater proportion of the identified hybrids had L. ewingii mtDNA. Geographical cline analyses showed a sharp transition in allele frequencies across the transect, and both the mtDNA and microsatellite data showed concordant cline centres, but were best supported by a model that allowed width to vary. Overall, the L. ewingii–L. paraewingi hybrid zone is best characterized as a tension zone, due to the narrow cline width, concordant genetic clines and low levels of hybridization.     

Risk assessment by crow phenotypes in a hybrid zone

Predation is one of the most selective forces in evolution and, thus, predation may select against hybrids in narrow hybrid zones. It may be possible that parental phenotypes and hybrids differ in their responses towards predators or humans. As predation is difficult to observe I used flight-initiation distance (FID) as a metric of risk assessment. FID is a measurable outcome of the trade-off between fleeing and remaining. Here, I tested whether hybrid and parent crow phenotypes (Corvus corone, Corvus cornix) from the hybrid zone in Eastern Germany differ in their FID. Further, I measured many environmental and social variables to control statistically for their influence on FID. I sampled 154 individuals (53 hooded crows, 54 carrion crows, and 48 hybrids) in the hybrid zone in eastern Germany. I calculated a general linear model using a stepwise backward procedure to establish a minimum model containing only significant variables that explained FID in crows. The variable phenotype (hooded, carrion, hybrid) was then added to the model. There were no differences in FID between hybrids and both parental phenotypes types, suggesting similar risk assessment. This suggests that hybrids may behave similarly in their decision to flee as their parent phenotypes, which, in turn, provides no evidence for a selective disadvantage. An additional analysis focusing on pure phenotypic flocks showed that hybrids in pure hybrid flocks had a lower FID than both parental species in pure flocks. This suggests that hybrids in pure hybrid flocks may be at a disadvantage.     

A natural hybrid ofMonophyllaea (Gesneriaceae) in the tropical rain forests of West Sumatra

A natural hybrid (2n = 21) between the parapatric rain forest speciesMonophyllaea hirtella (2n = 20) andM. horsfieldii (2n = 22) (Gesneriaceae) has been observed at Sg. Lubuk Paraku, Padang, W. Sumatra. The hybrids showed intermediary characters between the parental species in the inflorescence structure, flower size and colour, indumentum, chromosome numbers, and habitat. The 29% pollen fertility of a single hybrid specimen suggests that the genetical isolation between the parental species may break down and reproduction over subsequent generations may occur.     

Differential fate of plastid and mitochondrial genomes in Petunia somatic hybrids

Summary The chloroplast (cp) and mitochondrial (mt) DNAs of Petunia somatic hybrid plants, which were derived from the fusion of wild-type P. parodii protoplasts with albino P. inflata protoplasts, were analyzed by endonuclease restriction and Southern blot hybridization. Using ³²P-labelled probes that distinguished the two parental cpDNAs at a BamH1 site and at a HpaII site, only the P. parodii chloroplast genome was detected in the 10 somatic hybrid plants analyzed. To examine whether cytoplasmic mixing had resulted in rearrangement of the mitochondrial genome in the somatic hybrids, restriction patterns of purified somatic hybrid and parental mtDNAs were analyzed. Approximately 87% of those restriction fragments which distinguish the two parental genomes are P. inflata-specific. Restriction patterns of the somatic hybrid mtDNAs differ both from the parental patterns and from each other, suggesting that an interaction occurred between the parental mitochondrial genomes in the somatic fusion products which resulted in generation of the novel mtDNA patterns. Southern blot hybridization substantiates this conclusion. In addition, somatic hybrid lines derived from the same fusion product were observed to differ in mtDNA restriction pattern, reflecting a differential sorting-out of mitochondrial genomes at the time the plants were regenerated.     

Extensive mitochondrial heteroplasmy in hybrid water frog (Pelophylax spp.) populations from Southeast Europe

Water frogs of the genus Pelophylax (previous Rana) species have been much studied in Europe for their outstanding reproductive mechanism in which sympatric hybridization between genetically distinct parental species produces diverse genetic forms of viable hybrid animals. The most common hybrid is P. esculentus that carries the genomes of both parental species, P. ridibundus and P. lessonae, but usually transfers the whole genome of only one parent to its offsprings (hybridogenesis). The evolutionary cost of transfer of the intact genome and hence the hemiclonal reproduction is the depletion of heterozygosity in the hybrid populations. Pelophylax esculentus presents an excellent example of the long‐term sustained hybridization and hemiclonal reproduction in which the effects of the low genetic diversity are balanced through the novel mutations and periodic recombinations. In this study, we analyzed the mitochondrial (mt) and microsatellites DNA variations in hybrid Pelophylax populations from southern parts of the Pannonian Basin and a north–south transect of the Balkan Peninsula, which are home for a variety of Pelophylax genetic lineages. The mtDNA haplotypes found in this study corresponded to P. ridibundus and P. epeiroticus of the Balkan – Anatolian lineage (ridibundus–bedriagae) and to P. lessonae and a divergent lessonae haplotype of the lessonae lineage. The mtDNA genomes showed considerable intraspecific variation and geographic differentiation. The Balkan wide distributed P. ridibundus was found in all studied populations and its nuclear genome, along with either the lessonae or the endemic epeiroticus genome, in all hybrids. An unexpected finding was that the hybrid populations were invariably heteroplasmic, that is, they contained the mtDNA of both parental species. We discussed the possibility that such extensive heteroplasmy is a result of hybridization and it comes from regular leakage of the paternal mtDNA from a sperm of one species that fertilizes eggs of another. In this case, the mechanisms that protect the egg from heterospecific fertilization and further from the presence of sperm mtDNA could become compromised due to their differences and divergence at both, mitochondrial and nuclear DNA. The heteroplasmy once retained in the fertilized egg could be transmitted by hybrid backcrossing to the progeny and maintained in a population over generations. The role of interspecies and heteroplasmic hybrid animals due to their genomic diversity and better fitness compare to the parental species might be of the special importance in adaptations to miscellaneous and isolated environments at the Balkan Peninsula.     

ELECTROPHORETIC CONFIRMATION OF INTERSPECIFIC HYBRIDIZATION IN AESCULUS (HIPPOCASTANACEAE) AND THE GENETIC STRUCTURE OF A BROAD HYBRID ZONE

Within a broad (>200 km wide) hybrid zone involving three parapatric species of Aesculus, we observed coincident clines in allele frequency for 6 of 14 electrophoretic loci. The cooccurrence of alleles characteristic of A. pavia, A. sylvatica, and A. flava was used to estimate genetic admixtures in 48 populations involving various hybrids between these taxa in the southeastern United States. High levels of allelic polymorphism (up to 40% greater than the parental taxa) were observed in hybrid populations and also in some populations bordering the hybrid zone. A detailed analysis of a portion of the hybrid zone involving A. pavia and A. sylvatica revealed a highly asymmetrical pattern of gene flow, predominantly from Coastal Plain populations of A. pavia into Piedmont populations of A. sylvatica. Computer simulations were used to generate expected genotypic arrays for parental, F₁; and backcross individuals, which were compared with natural populations using a character index scoring system. In these comparisons, hybrid individuals could be distinguished from either parent, but F₁ and backcross progeny could not be distinguished from each other. Most hybrid populations were found to include hybrids and one of the parental taxa, but never both parents. Three populations appeared to be predominantly hybrids with no identifiable parental individuals. Hybrids occurred commonly at least 150 km beyond the range of A. pavia, but usually not more than 25 km beyond the range of A. sylvatica. Introgression, suggested by genetically hybrid individuals and significant gene admixtures of two or more species in populations lacking morphological evidence of hybridization, may extend the hybrid zone further in both directions. The absence of one or both parental species from hybrid populations implies a selective disadvantage to parentals in the hybrid zone and/or that hybridization has occurred through long-distance gene flow via pollen, primarily from A. pavia into A. sylvatica. Long-distance pollen movement in plants may generate hybrid zones of qualitatively different structure than those observed in animals, where gene flow involves dispersal of individuals.     

Extreme levels of intra-specific divergence among Cape Peninsula populations of the Cape galaxias,Galaxias zebratus Castelnau 1861, reveals a possible species complex

The Cape galaxias, Galaxias zebratus, is part of the paleao-endemic fauna characteristic of the south-western Cape, South Africa, and is the only galaxiid found in continental Africa. A 284-bp fragment of the cytochrome b region of the mtDNA was sequenced from 48 individual galaxiids, representing 10 populations from the Cape Peninsula. Five sequences, for four additional populations sampled at the extremes of the species range, were obtained from the literature. Analysis of cyt b mtDNA from these 14 populations of G. zebratus revealed five distinct and highly divergent lineages with low levels of intra-population mtDNA haplotype diversity. A new and distinct genetic lineage is described from the southern part of the Cape Peninsula. Estimates of genetic divergence between populations ranged from <1% to >17%. The observed level of sequence divergence represents the largest yet reported for any single fish species. The distribution of these lineages and their degree of sequence divergence refutes a model of isolation by distance. Results suggest that periods of low sea level may have been important in creating opportunities and alternative routes for dispersal and migration for Cape Peninsula populations.     

Origin of gynodioecy in Limnanthes: evidence from ecogeographic patterns of variation

Summary Evidence for hybrid origin of gene-cytoplasmic gynodioecy in Limnanthes douglasii is presented in terms of the parapatric distributions of putative parental taxa and the increased levels of genetic variation in gynodioecious populations. Attempts to produce gynodioecy through artificial hybridization between different accessions apparently failed due to the limited number of parental combinations used in making hybrids. Further studies are proposed on the hybrid origin model and on selective forces determining the fate of gynodioecy with its contributions to higher levels of hybridity and genetic variation.     

Introgressive hybridization between two Iberian endemic cyprinid fish: a comparison between two independent hybrid zones

Pseudochondrostoma duriense and Achondrostoma oligolepis are two Iberian endemic cyprinid fish species that occur in sympatry over most of their distribution range and that are suspected to hybridize in nature. Here, we employed a combination of mitochondrial and microsatellite markers to explore the extent of introgressive hybridization between these fishes. Two natural hybrid zones were identified in different river basins. Introgression was bi‐directional and both hybrid zones consisted mostly of parental genotypes/phenotypes (i.e. bimodal hybrid zones). Yet, they appeared to differ in the extent and direction of introgression, which supports the view that they constitute independent outcomes of different hybridization processes probably influenced by environmental features. Several discordances were found between mtDNA and microsatellite results, suggesting that this hybridization process has complex consequences and illustrating the importance of using independent markers to define accurately the hybrid status of individuals in the presence of high levels of backcrossing.     

Narrow hybrid zone between Moscow and Western Dvina chromosomal races and specific features of population isolation in common shrew <Emphasis Type="Italic">Sorex araneus</Emphasis> (Mammalia)

The contact zone between Moscow and Western Dvina chromosomal races of common shrew Sorex araneus L. at the south of the Valdai Hights was traced over a distance of 20 km. Within this, close to parapatric, contact zone of chromosomal races the width of sympatry zone was about 500 m (the narrowest among currently known hybrid zones), and the proportion of hybrids was 24.3%. It was shown that in bimodal hybrid zones between chromosomal races of common shrew the width of sympatry zones varied from 0.5 to 13 km. This width does not correlate with the cytogenetic features of the hybrids, and seems to be determined by competitive relations between the races. The hybrid proportion is determined by the type of hybrid heterozygosity, and decreased in the race sympatry zone from 33–40 to 21.5–25.2%. The decrease of the hybrid proportion can be associated with the abnormal fertility of either the first generation, or the backcross hybrids.     

Genomic versus chromosomal polytypy in studies of mitochondrial and nuclear DNA markers in the <Emphasis Type="Italic">Microtus arvalis</Emphasis> group

Common voles of the Microtus arvalis group distributed over the territory of European Russia are represented by three karyotypic categories, i.e., sympatric sibling species with 2n = 46 and 54, and two allopatric karyoforms in one of them, 2n = 46. For each category, molecular markers were found. For two 46-chromosome forms (arvalis and obscurus), iDNA was for the first time studied in karyotyped and non-karyotyped specimens for a parapatric hybrid zone, where high diversity of intermediate karyotypes was recorded. Preferential migration of the mitochondrial markers in arvalis and significant differences in the cline width for chromosomal and nuclear markers in obscurus were shown. The hybrid zone examined exhibited unusual combination of such features as the practically complete absence of “pure” representatives of the original parental forms and a clear deficiency of the first generation hybrids. The mtDNA divergence for the arvalis and obscurus karyoforms (4.6%) is comparable to the lowest limit for interspecies differences within the large and complex genus Microtus.     

Narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata: Asteraceae). II. Selection gradients and hybrid fitness

The dynamic equilibrium hypothesis proposes that hybrid zones are stabilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower fitness than either parental taxon, regardless of habitat. Hybrid big sagebrush (Artemisia tridentata ssp. tridentata × ssp. vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore loads, however, are not greater on the hybrids than on the parental subspecies. Browsing by mule deer is greatest on ssp. vaseyana in both hybrid zones. Incidence of galls is also greatest on ssp. vaseyana in one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on ssp. tridentata in one hybrid zone. Hybrids produce more flowers than either ssp. tridentata or ssp. vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybrid seeds germinate as well as those of ssp. tridentata and better than those of ssp. vaseyana. Thus, our data do not support the dynamic equilibrium hypothesis.     

Maternal effects and ecological divergence in aquatic plants: a case study in natural reciprocal hybrids between Potamogeton perfoliatus and P. wrightii

When parental taxa are adapted to different habitats, hybrid genotypes are often highly heterogeneous, such that habitat or ecological factors influence hybrid fate and ecological performance. Trait expression in hybrids is not always intermediate between the parents, but may instead be either parental‐like or extreme (transgressive) depending on genetic control of the phenotypes. Maternal effects arising from interspecific interaction between cytoplasmic and nuclear genomes are widely recognized as playing a role in character expression of natural hybrids. Such interaction often leads to hybrid sterility or inviability. When hybrids are viable, however, cytonuclear interaction may contribute to hybrid persistence through its influence on trait expression. To date, maternal influence on hybrid performance has been examined primarily in experimentally produced hybrids, or in natural hybrids without identification of the cross direction owing to difficulty in obtaining species‐specific molecular markers. In aquatic plants, many hybrids persist by extensive clonal growth and are important components of aquatic communities. Many such hybrids are known in Potamogeton (pondweeds), the largest aquatic genus. Because Potamogeton species are ecologically highly diverse and maternal lineages are readily distinguished using molecular markers, natural hybrids of Potamogeton are well‐suited for studies of maternal effects, especially those affecting vegetative performance. As a case study, we have focused on maternal effects on drought tolerance and depth distribution in the natural hybrid P. × anguillanus derived from the closely related species P. perfoliatus and P. wrightii.     

Phylogeography of the widespread African puff adder (Bitis arietans) reveals multiple Pleistocene refugia in southern Africa

Evidence from numerous Pan‐African savannah mammals indicates that open‐habitat refugia existed in Africa during the Pleistocene, isolated by expanding tropical forests during warm and humid interglacial periods. However, comparative data from other taxonomic groups are currently lacking. We present a phylogeographic investigation of the African puff adder (Bitis arietans), a snake that occurs in open‐habitat formations throughout sub‐Saharan Africa. Multiple parapatric mitochondrial clades occur across the current distribution of B. arietans, including a widespread southern African clade that is subdivided into four separate clades. We investigated the historical processes responsible for generating these phylogeographic patterns in southern Africa using species distribution modelling and genetic approaches. Our results show that interior regions of South Africa became largely inhospitable for B. arietans during glacial maxima, whereas coastal and more northerly areas remained habitable. This corresponds well with the locations of refugia inferred from mitochondrial data using a continuous phylogeographic diffusion model. Analysis of data from five anonymous nuclear loci revealed broadly similar patterns to mtDNA. Secondary admixture was detected between previously isolated refugial populations. In some cases, this is limited to individuals occurring near mitochondrial clade contact zones, but in other cases, more extensive admixture is evident. Overall, our study reveals a complex history of refugial isolation and secondary expansion for puff adders and a mosaic of isolated refugia in southern Africa. We also identify key differences between the processes that drove isolation in B. arietans and those hypothesized for sympatric savannah mammals.     

RESTRICTED GENE FLOW IN A MOVING HYBRID ZONE OF THE NEWTS TRITURUS CRISTATUS AND T. MARMORATUS IN WESTERN FRANCE

Two hybridizing species of newts, Triturus cristatus and T. marmoratus, with overlapping distributions show a parapatric distribution when surveyed in detail. The factors that govern the distribution of cristatus vs. marmoratus in the département (province) of Mayenne in western France are identified as forestation and relief. The parapatric hybrid zone running through Mayenne is narrow but widens to approximately 20 km in an area with mixed habitat. In this area most breeding sites are shared and F₁ hybrids form about 4% of the total population. Analysis of survey data collected about 30 years previously also shows an essentially parapatric distribution. Comparison of past and present distribution maps reveals that cristatus has superseded marmoratus over large areas in the south of Mayenne. An area where marmoratus replaced cristatus also exists, but it is more limited in size. Gene flow between cristatus and marmoratus is analyzed using 10 diagnostic genetic markers [9 protein loci and mitochondrial (mt) DNA]. In syntopic populations nuclear gene flow is bidirectional with a mean frequency of introgressed alleles (f) of 0.3%. In allotopic populations of cristatus and marmoratus gene flow is present in areas of species replacement (f = 0.3%), while gene flow appears to be absent in those areas that have been continuously occupied by a single species. At the biogeographic level, the presence or absence of introgression is paralleled by the persistence or absence, respectively, of pockets of cristatus–marmoratus syntopy. All F₁ hybrids possess the cristatus type mtDNA. This may be due to asymmetric interspecific mate choice and would explain the observed absence of introgression of the maternally inherited mtDNA genome in areas where cristatus replaced marmoratus. The cristatus–marmoratus hybrid zone bears characteristics of both the clinal (parapatric) hybrid zone model and the mosaic hybrid zone model. Such a mixed model—for which we propose the term “reticulate hybrid zone”—can be appreciated only if studied over a two-dimensional geographic area and also through time.