Barbara Mantovani

Satellite DNAs between selfishness and functionality: Structure, genomics and evolution of tandem repeats in centromeric (hetero)chromatin

Satellite DNAs (tandemly repeated, non-coding DNA sequences) stretch over almost all native centromeres and surrounding pericentromeric heterochromatin. Once considered as inert by-products of genome dynamics in heterochromatic regions, recent studies showed that satellite DNA evolution is interplay of stochastic events and selective pressure. This points to a functional significance of satellite sequences, which in (peri)centromeres may play some fundamental functional roles. First, specific interactions with DNA-binding proteins are proposed to complement sequence-independent epigenetic processes. The second role is achieved through RNAi mechanism, in which transcripts of satellite sequences initialize heterochromatin formation. In addition, satellite DNAs in (peri)centromeric regions affect chromosomal dynamics and genome plasticity. Paradoxically, while centromeric function is conserved through eukaryotes, the profile of satellite DNAs in this region is almost always species-specific. We argue that tandem repeats may be advantageous forms of DNA sequences in (peri)centromeres due to concerted evolution, which maintains high intra-array and intrapopulation sequence homogeneity of satellite arrays, while allowing rapid changes in nucleotide sequence and/or composition of satellite repeats. This feature may be crucial for long-term stability of DNA-protein interactions in centromeric regions.

HYBRIDOGENESIS AND ANDROGENESIS IN THE STICK-INSECT BACILLUS ROSSIUS-GRANDII BENAZZII (INSECTA, PHASMATODEA)

In northwestern Sicily interspecific hybrid females between Bacillus rossius and B. grandii benazzii (Insecta, Phasmatodea) are sympatric with facultatively parthenogenetic demes of the former and bisexual populations of the latter. Preliminary observations suggested that hybrid females are maintained by hybridogenetic reproduction, not by current F1 hybrid production nor through parthenogenesis. Being hybridogens, a complex of hemiclonal lineages, we informally refer to them as B. rossius‐grandii benazzii, according to Schultzs proposal. In this study B. rossius‐g. benazzii females were crossed with males of B. g. benazzii, B. g. grandii, B. g. maretimi, and B. rossius. Allozyme analysis of the progeny showed that the great majority of them were actually produced by hybridogenesis with a hemiclonal inheritance of the maternal B. rossius genotype (Brm) and actual syngamy with a sperm from the fathering male, so that Brm‐gbp, Brm‐ggp, Brm‐gmp, and Brm‐rp offspring were obtained in the respective crosses. All‐paternal progeny (androgenetics) were also produced (Bgbpgbp, Bgmpgmp, Brprp) and two gynogenetic descendants were observed. Cytological investigations on virgin eggs that failed to hatch revealed in most of them a haploid‐diploid blocked blastoderm; this rudimentary parthenogenesis appears to be an important prerequisite for further evolution of this hybridogen. Reproductive modes of descendants were also analyzed; although Brm‐gp hybrids are still able to reproduce by hybridogenesis, a progressive disruption of the hybridogenetic‐androgenetic system takes place in synthetic B. rossius (Brm‐rp, Brprp) and abundant thelytokous parthenogenetic offspring are obtained from females of androgenetic origin. The evolutionary role of these hybridogens appears to be linked to their shift towards parthenogenesis; this has apparently occurred in the southeastern Sicilian hybrid B. whitei (=B. rossius/g. grandii), which exhibits both hybridogenesis and parthenogenesis.

Taxonomy and phylogeny of north mediterranean Reticulitermes termites (Isoptera, Rhinotermitidae): a new insight

SummaryThe molecular characterisation of 18 new populations of Reticulitermes is here presented for COII and 16S genes; results are elaborated and compared to all available ones, at distances and gene trees (Maximum Parsimony, Maximum likelihood, Bayesian analysis) levels. Within the R. lucifugus complex, a subspecific rank of differentiation appears tenable for Italian (R. lucifugus lucifugus, R. lucifugus corsicus) and European (R. lucifugus banyulensis, R. lucifugus grassei) taxa; a subspecific differentiation emerges also for the Sicilian samples. The existence of two different entities in the area formerly defined as inhabited by R. balkanensis is demonstrated; The north-eastern Italian Reticulitermes sp. is found to be more widely distributed in northern and south-eastern Italy and shows a close relationship to the sample from Peloponnese; the GenBank sample from continental Greece, on the contrary, appears more related to other eastern taxa such as R. lucifugus from Turkey and R. clypeatus from Israel; the distribution and differentiation of eastern Reticulitermes taxa are explained through the role that southern Balkans should have played as glacial refuge. Present data also evidences instances of anthropic involvement in taxa distribution; for one of these, the importation of at least a family group is taken into account.

Polymerase chain reaction amplification of the Bag320 satellite family reveals the ancestral library and past gene conversion events in Bacillus rossius (Insecta Phasmatodea).

Polymerase chain reaction amplifications of genomic DNA in 17 individuals of bisexual and parthenogenetic populations of three subspecies of Bacillus rossius (Insecta Phasmatodea) revealed that the species still harbours the whole variability of the ancestral Bag320 satellite family, since monomers of all non-hybrid Bacillus taxa plus private sequences occur in it. Bag320 monomers had not been rescued as a major satellite component in B. rossius, but possibly represent the remnant of a set of diverging sequences present in the Bacillus ancestor. Following the library hypothesis, these monomer variants have been differently amplified along the evolutionary pathways leading to present taxa in agreement with the mitochondrial phylogeny of the genus. The putative converted tracts observed are explained as the results of past gene conversion events.

Molecular taxonomy and phylogeny of the ‘living fossil’ lineages Triops and Lepidurus (Branchiopoda: Notostraca)

Mantovani B. Cesari M. & Scanabissi F. (2004). Molecular taxonomy and phylogeny of the ‘living fossil’ lineages Triops and Lepidurus (Branchiopoda: Notostraca). —Zoologica Scripta, 33, 367–374.

Mitochondrial evolutionary rate and speciation in termites: data on European Reticulitermes taxa (Isoptera, Rhinotermitidae)

Summary.The rate of mitochondrial DNA evolution and the speciation pattern in relation to glacial periods are tested in the European taxa of the eusocial genus Reticulitermes. The linearized tree obtained from cytochrome oxidase II sequences and a geological event calibration shows a substitution rate 100-fold higher than that usually applied for insect mitochondrial DNA. An accelerated rate of evolution has also been observed in social Vespidae (Hymenoptera); we therefore suggest the involvement of eusociality in mediating gene pool drift. The role of the last ice age in speciation pattern of Reticulitermes taxa is supported by molecular data, but a four refugia model better explains genetic diversity, phyletic relationships and present-day distribution of these termites.

Mate recognition and gamete cytology features allow hybrid species production and evolution in Bacillus stick insects

Abstract The increasing number of recognized hybrid unisexual complexes among invertebrate and vertebrate animals has promoted investigations about their composition and origin. Morphological, karyological and genetic (protein and DNA) analyses clearly show that, owing to their persistence and incomplete reproductive isolation from ancestors, several all‐female complexes are much more diversified than generally assumed and that they may also have an evolutionary role. Here the case of the stick‐insects of the genus Bacillus is reported in some detail. This holomediterranean genus comprises three well differentiated species that in Sicily have hybridized repeatedly. The Bacillus mate‐recognition system has not followed the species‐specific differentiation of the allozyme‐coding loci, allowing interspecific crosses to occur in areas of species sym‐patry with the production of two hybridogens, a corresponding allodiploid parthenogen and a trihybrid triploid parthenogenetic species. Hybridogenetic females eli...

Allozyme analysis and phyletic relationships of two new stick‐insects from north‐west Sicily: Bacillus grandii benazzii and B. rossius‐grandii benazzii (Insecta Phasmatodea)

Two new Sicilian stick‐insects have been discovered within the genus Bacillus. Evidence concerning the subspecific differentiation of Bacillus grandii benazzii and the hybrid constitution of B. rossius‐grandii benazzii by means of allozyme analysis is given, and is consistent with morphological and karyological data on the differentiation of these two north‐west Sicilian morphs from south‐eastern B. g. grandii and B. whitei. B. g. benazzii is more polymorphic than B. g. grandii and B. rossius‐g. benazzii embodies its genetic variability, thus differing sharply from the south‐eastern parallel hybrid B. whitei ( = B. rossius × B. g. grandii). Reproductive mechanisms also appear to be different in the two interspecific hybrids, involving parthenogenesis in the latter and hybridogenesis in the former. Finally, the phyletic relationships among all Sicilian Bacillus taxa are summarized in a revised scheme, which also takes into account new evidence that B. atticus has contributed to hybrid constitution of the triploid B. lynceorum.

Updating of systematics and speciation mechanisms of Bacillus (Insecta, Phasmatodea)

Abstract Following the recent discovery of seven new species, ten specific taxa are now recognized within the genus Bacillus: 1. B. rossius; 2. B. grandii; 3. B. whitei; 4. B. lynceorum; 5. B. atticus; 6. B. diplocarius; 7. B. carius; 8. B. rhodius; 9. B. cyprius; 10. B. creticus. Furthermore, eight subspecies have been defined within B. rossius. The distribution and the ecology of the species are summarized. On the basis of ootaxonomy, karyology, electrophoretic allozyme analysis and vitellin comparisons, the phyletic relationships among taxa within the genus are suggested, with particular emphasis on the natural thely‐tokous hybrids, B. whitei diploid and the triploid B. lynceorum. The main evolutionary mechanisms at work here appear to be: interspecific hybridization, polyploidy, chromosome repatterning and the switch to parthenogenetic reproduction. Gradual genetic differentiation has produced subspecific taxa in B. rossius.

Phylogeny and karyotype evolution of the Iberian Leptynia attenuata species complex (Insecta Phasmatodea).

An in-depth analysis of the Leptynia attenuata species complex has been performed by cytochrome oxidase subunit 2 (cox2) gene sequencing as well as karyotype and allozyme analysis. The whole set of data allows to largely resolve the taxonomy of the group and suggests an overall trend of chromosomal repatternings through a progressive reduction of the chromosome number. A previously suggested new species has been also confirmed on a genetic basis. Data are discussed in order to depict a phylogenetic and phylogeographic scenario fitting the observed genetic relationships between the different species of the group. Chromosome rearrangements are proposed as the major speciation driving force within the group and androgenetic reproduction is suggested as a shortcut to overcome the problem of fixing chromosomal rearrangements that are strongly underdominant in heterozygotes.