Valerio Scali

Gender-associated mitochondrial DNA heteroplasmy in the venerid clam Tapes philippinarum (Mollusca Bivalvia)

Abstract This paper provides evidence of a gender-associated mitochondrial DNA (mtDNA) heteroplasmy in the clam Tapes philippinarum. The pattern of tissue distribution of the two observed mtDNA types (referred to as M and F) parallels that of the doubly uniparental inheritance system of Mytilus. The mitochondrial genetic features of the clam are related to a different rate of evolution of the M- and F-type mtDNAs. Since clams are known to be phylogenetically very distant from mussels, the present findings support the hypothesis that the mechanism producing gender-associated heteroplasmy should be considered an ancestral character in the Bivalvia.

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.

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.

The Leptynia hispanica species complex (Insecta Phasmida): polyploidy, parthenogenesis, hybridization and more

The Leptynia hispanica stick insect species complex includes bisexuals, triploid and tetraploid parthenogenetic populations, suggesting that polyploidy has played a central role in the evolution of this complex. An analysis of karyotype, mitochondrial DNA (cox2) and nuclear DNA (ef1‐α) markers was carried out to clarify phylogenetic relationships and microevolutionary/phylogeographical patterns of the L. hispanica complex. Our analyses suggested a subdivision of bisexual populations into four groups, tentatively proposed as incipient species. Moreover, triploids and tetraploids showed two independent origins, the latter being more ancient than the former. From ef1‐α analysis, triploids showed hybrid constitution, while the hybrid constitution of tetraploids is likely, but more data are needed. We suggest that L. hispanica is a case of ‘geographical parthenogenesis’ with parthenogenetic strains colonizing large peripheral ranges, and bisexuals confined to glacial refuge areas. Moreover, the age, wide distribution and competitive advantage of polyploids over diploids, demonstrate their significance in the evolution of the L. hispanica species complex.

Metasexual Stick Insects: Model Pathways to Losing Sex and Bringing It Back

This chapter illustrates the variety of parthenogenetic mechanisms within three genera of phasmids (stick insects) occurring around the Mediterranean and the close ties between sexuals and asexuals. The link between parthenogenesis and polyploidy appears to provide an escape from cytogenetic disruption. When compared to diploids, both auto-polyploids and allo-polyploids can take advantage of higher mutation rates and the resulting increase of heterozygosity; they can also escape the accumulation of mutations. Detailed investigations of the karyotype structure, reproduction modes and phylogenetic relationships reveal a wide variety of reproductive modes including repeated backcrosses to parental species. Diploid hybrids can also reproduce hemiclonally, with one parental genome being eliminated and the hybrid being produced anew in each generation. However, also the reversal route from hybrids to the fathering species occurs through androgenesis, which resembles sexual reproduction with most of its genetic traits. All of these reproductive modes can be considered as patterns of “reticulate” evolution. Automicts, apomicts and hybrids with different amounts of recombination and levels of polyploidy (from diploids to tetraploids) have been found. The genetic diversity in these asexuals is kept high in mitochondrial DNA, rDNA cistrons, satellite DNA and coding genes. Other addressed issues, clearly linked to phasmid parthenogenesis are the peculiarities of spindle structure and the microtubule and γ-tubulin patterns for the egg development: these might be one of the reasons why phasmids are so exceptional in their variety of reproductive modes.

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...

ANDROGENETICS AND TRIPLOIDS FROM AN INTERACTING PARTHENOGENETIC HYBRID AND ITS ANCESTORS IN STICK INSECTS

Populations of unisexual organisms are often assumed to be genetically invariant (clones) and destined to a short existence on an evolutionary timescale. Unisexual organisms are most often obligate parthenogens and, by definition, ought to be completely isolated reproductively from related bisexual organisms. The assumption of complete reproductive isolation between amphimictic ancestors and thelytokous hybrids is common to most hypotheses on the evolution of sex and its adaptive significance. Stick insects of the genus Bacillus however provide evidence for reproductive interactions between allodiploid parthenogens and their ancestors, because pure species progeny (androgenetics) and triploid descendants are produced. These findings demonstrate that, through androgenesis, offspring of parthenogenetic hybrid females can contribute specimens of both sexes to the fathering species when fertilized by syntopic ancestral males and the parthenogenetic egg of strictly clonal females, when fertilized, allows a third genome to be added to the allodiploid chromosome set. These triploid genomes promote further genetic diversification and evolution of the unisexual populations through the formation of new clones by recombination during the changed maturation mode of allotriploid eggs. All this argues for much more complex breeding systems and evolutionary pathways than are usually assumed for hybrid unisexual organisms.

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.

Heat stress and age induced maternal effects on wing size and shape in parthenogenetic Drosophila mercatorum

Maternal effects on progeny wing size and shape in a homozygous parthenogenetic strain of Drosophila mercatorum were investigated. The impact of external maternal factors (heat stress) and the impact of internal maternal factors (different maternal and grand maternal age) were studied. The offspring developed under identical environmental conditions, and due to lack of genetic variation any phenotypic difference among offspring could be ascribed to maternal effects. Wing size was estimated by centroid size, shape was analysed with the Procrustes geometric morphometric method and variation in landmark displacement was visualized by principal component analysis. Both kinds of maternal effects had a significant impact on progeny wing size and shape. Maternal heat stress led to the same pattern of response in size and shape among the progeny, with increased difference between the control group and progeny from heat stressed flies in both size and shape with increased maternal heat stress temperature. The effects of maternal age, however, led to different responses in size and shape between the different progeny groups. The observed variation in landmark displacements was similar, and in both cases mainly associated with shape differences of the posterior part of the wing. Finally, our results suggest that maternal effect has some evolutionary implications by altering the genetic correlations among traits, which can affect the response to selective pressures.