George C. Rodakis

MITOCHONDRIAL PHYLOGEOGRAPHY OF THE LAND SNAIL ALBINARIA IN CRETE: LONG-TERM GEOLOGICAL AND SHORT-TERM VICARIANCE EFFECTS

The land snail genus Albinaria exhibits an extreme degree of morphological differentiation in Greece, especially in the island of Crete. Twenty‐six representatives of 17 nominal species and a suspected hybrid were examined by sequence analysis of a PCR‐amplified mitochondrial DNA fragment of the large rRNA subunit gene. Maximum parsimony and neighbor‐joining phylogenetic analyses demonstrate a complex pattern of speciation and differentiation and suggest that Albinaria species from Crete belong to at least three distinct monophyletic groups, which, however, are not monophyletic with reference to the genus as a whole. There is considerable variation of genetic distance within and among “species” and groups. The revealed phylogenetic relations do not correlate well with current taxonomy, but exhibit biogeographical coherence. Certain small‐ and large‐scale vicariance events can be traced, although dispersal and parapatric speciation may also be present. Our analysis suggests that there was an early and rapid differentiation of Albinaria groups across the whole of the range followed by local speciation events within confined geographical areas.

Evolution of two major chorion multigene families as inferred from cloned cDNA and protein sequences

Complete or partial sequences are reported from six chorion cDNA clones of the silkmoth Antheraea polyphemus. The proteins encoded belong to the two major chorion protein classes, A and B, each of which is encoded by a multigene family. The sequence comparisons define some major features of the families and suggest how these genes may be evolving. Deletions and insertions might be involved in expanding or contracting internally repetitive regions. Sequence divergence is localized, thus defining sequence domains of distinct evolutionary properties and presumably distinct functions.

No evidence for presence of maternal mitochondrial DNA in the sperm of Mytilus galloprovincialis males

Species of the mussel family Mytilidae have a special mitochondrial DNA (mtDNA) transmission system, known as doubly uniparental inheritance (DUI), which consists of a maternally inherited (F) and a paternally inherited (M) mitochondrial genome. Females are normally homoplasmic for the F genome and males are heteroplasmic mosaics, with their somatic tissues dominated by the maternal and their gonads dominated by the paternal genome. Several studies have indicated that the maternal genome may often be present in the male germ line. Here we report the results from the examination of mtDNA in pure sperm from more than 30 males of Mytilus galloprovincialis. In all cases, except one, we detected only the M genome. In the sperm of one male, we detected a paternal genome with an F-like primary sequence that was different from the sequence of the maternal genome in the animals somatic tissues. We conclude that the male germ line is protected against invasion by the maternal genome. This is important because fidelity of gamete-specific transmission of the two mitochondrial genomes is a basic requirement for the stability of DUI.

Novel Features of Metazoan mtDNA Revealed from Sequence Analysis of Three Mitochondrial DNA Segments of the Land Snail Albinaria turrita (Gastropoda: Clausiliidae)

The mitochondrial DNA (mtDNA) size of the terrestrial gastropod Albinaria turrita was determined by restriction enzyme mapping and found to be approximately 14.5 kb. Its partial gene content and organization were examined by sequencing three cloned segments representing about one-fourth of the mtDNA molecule. Complete sequences of cytochrome c oxidase subunit II (COII), and ATPase subunit 8 (ATPase8), as well as partial sequences of cytochrome c oxidase subunit I (COI), NADH dehydrogenase subunit 6 (ND6), and the large ribosomal RNA (IrRNA) genes were determined. Nine putative tRNA genes were also identified by their ability to conform to typical mitochondrial tRNA secondary structures. An 82-nt sequence resembles a noncoding region of the bivalve Mytilus edulis, even though it might contain a tenth tRNA gene with an unusual 5-nt overlap with another tRNA gene. The genetic code of Albinaria turrita appears to be the same as that of Drosophila and Mytilus edulis. The structures of COI and COII are conservative, but those of ATPase8 and ND6 are diversified. The sequenced portion of thelrRNA gene (1,079 nt) is characterized by conspicuous deletions in the 5′ and 3′ ends; this gene represents the smallest coelomate IrRNA gene so far known. Sequence comparisons of the identified genes indicate that there is greater difference between Albinaria and Mytilus than between Albinaria and Drosophila. An evolutionary analysis, based on COII sequences, suggests a possible nonmonophyletic origin of molluskan mtDNA. This is supported also by the absence of the ATPase8 gene in the mtDNA of Mytilus and nematodes, while this gene is present in the mtDNA of Albinaria and Cepaea nemoralis and in all other known coelomate metazoan mtDNAs.

The Control Region of Maternally and Paternally Inherited Mitochondrial Genomes of Three Species of the Sea Mussel Genus Mytilus

Species of the mussel genus Mytilus possess maternally and paternally transmitted mitochondrial genomes. In the interbreeding taxa Mytilus edulis and M. galloprovincialis, several genomes of both types have been fully sequenced. The genome consists of the coding part (which, in addition to protein and RNA genes, contains several small noncoding sequences) and the main control region (CR), which in turn consists of three distinct parts: the first variable (VD1), the conserved (CD), and the second variable (VD2) domain. The maternal and paternal genomes are very similar in gene content and organization, even though they differ by >20% in primary sequence. They differ even more at VD1 and VD2, yet they are remarkably similar at CD. The complete sequence of a genome from the closely related species M. trossulus was previously reported and found to consist of a maternal-like coding part and a paternal-like and a maternal-like CR. From this and from the fact that it was extracted from a male individual, it was inferred that this is a genome that switched from maternal to paternal transmission. Here we provide clear evidence that this genome is the maternal genome of M. trossulus. We have found that in this genome the tRNAGln in the coding region is apparently defective and that an intact copy of this tRNA occurs in the CR, that one of the two conserved domains is missing essential motifs, and that one of the two first variable domains has a high rate of divergence. These features may explain the large size and mosaic structure of the CR of the maternal genome of M. trossulus. We have also obtained CR sequences of the maternal and paternal genomes of M. californianus, a more distantly related species. We compare the control regions from all three species, focusing on the divergence among genomes of different species origin and among genomes of different transmission routes.

A complex set of early chorion DNA sequences from Bombyx mori

A follicular cDNA library from Bombyx mori (576 clones) was screened to obtain a representative sample of chorion sequences differentially expressed during the early period of choriogenesis. Ten selected sequences were characterized by Northern analysis and by dot blots of stage-specific RNA. Five of these sequences (represented in the library by 24 clones) had the developmental specificity and transcript size expected of early chorion components; two sequences (37 clones) had the developmental specificity expected of middle and late chorion components; and three sequences (16 clones) were presumed to be non-chorion, since they are constitutive or disappear at the beginning of choriogenesis. The five recovered early chorion cDNA components have been sequenced, and define three distinct subfamilies of the chorion B multigene family, and two C type families. The complexity of the early chorion genes in B. mori is discussed.

Diversity in a chorion multigene family created by tandem duplications and a putative gene-conversion event

SummaryTwo families of high-cysteine chorion proteins inBombyx mori are encoded in 15 tandemly arranged nonidentical gene pairs. It is assumed that this locus arose by duplication with subsequent sequence divergence. We have compared DNA sequences from two such neighboring pairs of genes in an attempt to understand the manner in which diversity has been generated and/or removed. A high level of sequence identity (91%–99%) was found between the repeats throughout the transcribed and flanking regions, with two significant exceptions. First, in the DNA segment encoding a conserved region of the chorion proteins, ten substitutions were detected in a 39-base-pair region. This localized region of high variability would suggest an intergene conversion-like event. Second, a length difference of 141 base pairs was detected in a region encoding the carboxy-terminal arm of the protein. This difference can be explained by three separate reiterations of single codons (3 base pairs) separated in time by duplication or triplication events.

Evolution of a multigene family of chorion proteins in silkmoths.

The evolution of the A family of chorion genes was examined by comparing new protein and DNA sequences from the silkmoths Antheraea pernyi and Bombyx mori with previously known sequences from Antheraea polyphemus. The comparisons indicated that the A family and its major subfamilies are ancient and revealed how parts of the genes corresponding to distinct regions of the protein structure have evolved, both by base substitutions and by segmental reduplications and deletions.

Developmental Control and Evolution in the Chorion Gene Families Of Insects

Publisher Summary Extensive sequence information is now available for the major chorion components in both moths and flies. These proteins have proved to be numerous but belong to a small number of families. In silk moths, such as the cultivated Bombyx mori and the wild oak silk moth Antheraea polyphemus , more than 100 major chorion components of low molecular weight can be resolved by two-dimensional gel electrophoresis. The proteins are classified according to the distinctive properties of three regions: the central domain and the flanking NH 2 - and COOH-terminal ends (left and right arms). Two fundamental groups of proteins are evident, α or β. Each group shows identities ranging from less than 50% to more than 99% and is accordingly subdivided into multiple families, subfamilies, types, and copies. The α- and β-protein groups are defined by the central domain sequences. The β proteins are highly similar in the 48-residue central domain. This high conservation most probably reflects a universal requirement for a compact secondary structure in this domain, apparently consisting of eight very short antiparallel β-sheet strands alternating with β-turns.

Homologous Recombination between Highly Diverged Mitochondrial Sequences: Examples from Maternally and Paternally Transmitted Genomes

Homologous recombination is restricted to sequences of low divergence. This is attributed to the mismatch repairing system (MMR), which does not allow recombination between sequences that are highly divergent. This acts as a safeguard against recombination between nonhomologous sequences that could result in genome imbalance. Here, we report recombination between maternal and paternal mitochondrial genomes of the sea mussel, whose sequences differ by >20%. We propose that the strict maternal inheritance of the animal mitochondrial DNA and the ensuing homoplasmy has relieved the MMR system of the animal mitochondrion from the pressure to tolerate recombination only among sequences with a high degree of similarity.