Christina Spolsky

Phylogeography of the fire-bellied toads Bombina: independent Pleistocene histories inferred from mitochondrial genomes.

The fire‐bellied toads Bombina bombina and Bombina variegata, interbreed in a long, narrow zone maintained by a balance between selection and dispersal. Hybridization takes place between local, genetically differentiated groups. To quantify divergence between these groups and reconstruct their history and demography, we analysed nucleotide variation at the mitochondrial cytochrome b gene (1096 bp) in 364 individuals from 156 sites representing the entire range of both species. Three distinct clades with high sequence divergence (K2P = 8–11%) were distinguished. One clade grouped B. bombina haplotypes; the two other clades grouped B. variegata haplotypes. One B. variegata clade included only Carpathian individuals; the other represented B. variegata from the southwestern parts of its distribution: Southern and Western Europe (Balkano–Western lineage), Apennines, and the Rhodope Mountains. Differentiation between the Carpathian and Balkano–Western lineages, K2P ∼ 8%, approached interspecific divergence. Deep divergence among European Bombina lineages suggests their preglacial origin, and implies long and largely independent evolutionary histories of the species. Multiple glacial refugia were identified in the lowlands adjoining the Black Sea, in the Carpathians, in the Balkans, and in the Apennines. The results of the nested clade and demographic analyses suggest drastic reductions of population sizes during the last glacial period, and significant demographic growth related to postglacial colonization. Inferred history, supported by fossil evidence, demonstrates that Bombina ranges underwent repeated contractions and expansions. Geographical concordance between morphology, allozymes, and mtDNA shows that previous episodes of interspecific hybridization have left no detectable mtDNA introgression. Either the admixed populations went extinct, or selection against hybrids hindered mtDNA gene flow in ancient hybrid zones.

Widespread unidirectional transfer of mitochondrial DNA: a case in western Palaearctic water frogs

Interspecies transfer of mitochondrial (mt) DNA is a common phenomenon in plants, invertebrates and vertebrates, normally linked with hybridization of closely related species in zones of sympatry or parapatry. In central Europe, in an area north of 48°N latitude and between 8° and 22°E longitude, western Palaearctic water frogs show massive unidirectional introgression of mtDNA: 33.7% of 407 Rana ridibunda possessed mtDNA specific for Rana lessonae. By contrast, no R. lessonae with R. ridibunda mtDNA was observed. That R. ridibunda with introgressed mitochondrial genomes were found exclusively within the range of the hybrid Rana esculenta and that most hybrids had lessonae mtDNA (90.4% of 335 individuals investigated) is evidence that R. esculenta serves as a vehicle for transfer of lessonae mtDNA into R. ridibunda. Such introgression has occurred several times independently. The abundance and wide distribution of individuals with introgressed mitochondrial genomes show that R. lessonae mt genomes work successfully in a R. ridibunda chromosomal background despite their high sequence divergence from R. ridibunda mtDNAs (14.2–15.2% in the ND2/ND3 genes). Greater effectiveness of enzymes encoded by R. lessonae mtDNA may be advantageous to individuals of R. ridibunda and probably R. esculenta in the northern parts of their ranges.

Mitochondrial DNA variation in the hybridizing fire‐bellied toads, Bombina bombina and B. variegata

Using five restriction enzymes, geographical variation of mitochondrial DNA (mtDNA) in Bombina bombina and B. variegata was studied in samples from 20 locations. Each restriction enzyme produced a species‐specific fragment pattern. B. bombina haplotypes A and B were closely related to each other. In contrast, haplotypes A and B of B. variegata formed two distinct lineages. A very distinctive haplotype (C) was found in the Carpathian Mountains, whereas two other haplotypes, D and E (differing by a single AvaI site), were present in western Europe and the Balkans, respectively. Populations polymorphic for haplotypes D and E occurred in the central Balkans where the haplotypes could replace each other clinally. mtDNA sequence divergence between B. bombina and B. variegata was estimated as 6.0–8.1% and 4.7–5.2% between type C and types D/E of B. variegata. The latter divergence is contrary to allozyme and morphological data that place the western and Carpathian B. v. variegata together (Neis D = 0.07) and separate them from the Balkan subspecies B. v. scabra (Neis D = 0.18). Broad interspecific correlation among morphology, allozymes and mtDNA types in European fire‐bellied toads argues that, despite continuous hybridization (interrupted perhaps during Pleistocene glacial maxima), little or no mtDNA introgression between the species has occurred outside the narrow hybrid zones that separate these parapatric species.

Oncomelania hupensis (Gastropoda: Rissooidea) in eastern China: molecular phylogeny, population structure, and ecology.

The rissooidean snail genus Oncomelania is of medical interest as various taxa are hosts for the human blood fluke Schistosoma and the lung fluke Paragonimus; because of close co-evolved host-parasite-relationships, snail diversity may reflect parasite diversity. There is a considerable amount of confusion regarding the identity of smooth- and ribbed-shelled populations of Oncomelania hupensis in eastern China. We therefore studied the genetic variation, population structure, phylogenetic relationships and ecology of five smooth- and five ribbed-shelled populations in Hubei, Hunan, Anhui, Zhejiang, and Jiangsu provinces. Based on sequencing data of a fragment of the mitochondrial gene for cytochrome oxidase I from 80 individuals, we found little genetic variability within the ingroup-individuals studied here (average pi=0.01922). Moreover, within the ingroup, smooth-shelled individuals cluster together with ribbed-shelled individuals. We therefore consider all smooth- and ribbed-shelled populations of Oncomelania throughout the lower Yangtze River basin to belong to the subspecies O. hupensis hupensis. Our data indicate that ribbing in O. h. hupensis is associated with the annual floods of the Yangtze River. The greatest haplotype (d(H)) and nucleotide diversities (pi) are found in aggregates of ribbed-shelled snails along areas of the Yangtze River drainage subject to flooding. In areas not affected by flooding, the shells are smooth and genetic diversity decreases significantly.

Gynogenetic reproduction in hybrid mole salamanders (Genus Ambystoma)

Ambystoma platineum, a unisexual clonal triploid taxon of mole salamander, originated by hybridization between the Mendelian species A. jeffersonianum and A. laterale. Studies of lampbrush chromosomes indicated that A. platineum reproduces gynogenetically, that is, sperm from a sexual host species is required to activate egg development but makes no genetic contribution to the developing embryo. Nevertheless, electrophoretic diversity in populations of some hybrid Ambystoma suggested continual in situ recreation of unisexual hybrids and bidirectional gene exchange between the parental species and the hybrids. A. platineum usually lives with, and is sexually dependent on, one of its parental species, A. jeffersonianum. In central Indiana, however, A. platineum populations have shifted their host dependency to A. texanum. Such A. texanum‐dependent populations of A. platineum provide an almost ideal system for studying reproductive mode in A. platineum, because both replacement of a jeffersonianum or laterale genome of A. platineum by a texanum genome, and movement of genes from A. platineum to the host species, A. texanum, would be readily detected by electrophoretic markers. Our samples of A. texanum provided no evidence for the transfer of jeffersonianum or laterale genes into A. texanum. Similarly, among 32 A. platineum sampled from six localities in east‐central Illinois and central Indiana, we find no texanum alleles, and thus no evidence for genome replacement. The one diploid hybrid individual contained only a jeffersonianum and a laterale genome; because of the absence of either parental species from these populations, this hybrid could only have come from a diploid ovum produced by A. platineum. Both morphometric and electrophoretic results for the two tetraploid individuals indicate that they resulted from fertilization of triploid oocytes of A. platineum by sperm of A. texanum. Because genome replacement in A. texanum‐dependent populations of A. platineum is irreversible, the persistence of A. platineum in A. texanum‐dependent populations demonstrates conclusively that the major mode of reproduction in A. platineum populations is clonal: A. platineum produces mainly triploid eggs that develop gynogenetically.

TWO DATA SETS: ALTERNATIVE EXPLANATIONS AND INTERPRETATIONS

For those biologists interested in the evolutionary origin of the higher taxa of organisms, the origin of eukaryotic cells and their organelles is one of the most puzzling events in the history of the living world. For those biologists studying the eukaryotic organelles themselves, an understanding of the origin of the organelles is an essential complement to a description of their structure and function. Although two competing hypotheses concerning the origin of eukaryotic organelles (at least of the chloroplasts and mitochondria) have been available since 1890,*~ it is only in the last 30 years that basic data bearing on these hypotheses have begun to accumulate. One of these basic facts is our discernment of the fundamental structural differences between eukaryotes and prokaryotes. A traditional view of the origin of cellular organelles is that they arose autogenously, by small evolutionary steps, within some prokaryotic lineage. Although this view is traditional, that is not evidence for or against its correctness. The traditional view may, however, involve a rather unthinking acceptance of the idea that there has been a single origin of life, followed by subsequent divergence. Such assumptions should never remain unchallenged. A worthy challenge to this traditional view is posed by the hypothesis that eukaryotic organelles arose by endosymbiosis, a hypothesis picked up, dusted off, shaken into conformity with a lot of modern data, and stated vigorously by Lynn Margulis in her book, Origin of Eukaryotic Cells.1r This hypothesis has been a fruitful scaffolding on which to hang much of the data on structure and function of cellular organelles and their constituents, and is consonant with much new data on the relations of prokaryotic groups. One need only read widely in several journals, or examine many of the presentations at this symposium, to see the effectiveness of this hypothesis and to realize the wide acceptance it has won. And yet history repeats itself. Rather than being an alternative hypothesis for challenging the tradition, endosymbiosis has become a new tradition in itself. It is sometimes taken as proven, at least for chloroplast^,^^ and often appears to be almost unthinkingly accepted. We find, however, that many of the data so far adduced in favor of endosymbiosis are equally supportive of the earlier traditional view.

Comparative Analysis of Mitochondrial Genomes in Bombina (Anura; Bombinatoridae)

The complete mitochondrial genomes of two basal anurans, Bombina bombina and B. variegata (Anura; Bombinatoridae), were sequenced. The gene order of their mitochondrial DNA (mtDNA) is identical to that of canonical vertebrate mtDNA. In contrast, we show that there are structural differences in regulatory regions and protein coding genes between the mtDNA of these two closely related species. Corrected sequence divergence between the mtDNA of B. bombina and B. variegata amounts to 8.7% (2.3% divergence in amino acids). Comparisons with two East Asian congeners show that the control region contains two repeat regions, LV1 and LV2, present in all species except for B. bombina, in which LV2 has been secondarily lost. The rRNAs and tRNAs are characterized by low nucleotide divergence. The protein coding genes are considerably more disparate, although functional constraint is high but variable among genes, as evidenced by dN/dS ratios. A mtDNA phylogeny established the distribution of autapomorphic nonsynonomous substitutions in the mitogenomes of B. bombina and B. variegata. Nine of 98 nonsynonomous substitutions led to radical amino acid replacements that may alter mitochondrial protein function. Most radical substitutions were found in ND2, ND4, or ND5, encoding mitochondrial subunits of complex I of the electron transport system. The extensive divergence between the mitogenomes of B. bombina and B. variegata is discussed in terms of its possible role in impeding gene flow in natural hybrid zones between these two species.