New DAPI and fish findings on egg maturation processes in related hybridogenetic and parthenogenetic Bacillus hybrids (insecta, phasmatodea)

Abstract

Bacillus stick insects have proved adequate for studying a wide array of reproductive modes: sexual, parthenogenetic, hybridogenetic, androgenetic. Hybridogenetic strains (B. rossius-grandii) were thought to discard the paternal "grandii" haploset during first meiotic division and keep the "rossius" hemiclone, whereas the clonal B. whitei (=rossius/grandii) would maintain its hybrid structure by fusing back two nonsister nuclei-each derived from previously segregated heterospecific complements-by the end of the 2(nd) meiotic division. New investigations on laid eggs and ovariole squashes, either DAPI stained or FISH labeled, revealed that in hybridogens the "grandii" set is excluded from the germ line prior to meiosis and that a DNA extra-synthesis should occur to produce hemiclonal eggs after two cytologically normal meiotic divisions. On the other hand, in B. whitei eggs no genome segregation appears to occur and an intrameiotic DNA extra-synthesis must take place to produce 2n tetrachromatidic oocytes I; these divide twice and give unreduced clonal eggs. The new findings bring hybridogenetic oogenesis of Bacillus to be coincident with that of the known hemiclonal organisms and point to an independent onset of B. whitei from hemiclonal strains. In addition, B. whitei gains a closer resemblance to B. lynceorum owing to the sharing of a cytologically identical egg maturation mechanism, of the same maternal ancestor and of peculiar chromosomal features. It is here suggested that B. lynceorum originated from the incorporation of an "atticus" genome into a B. whitei egg, according to a pathway of repeated hybridization often occurred with other polyploid hybrids.

Linkage between sexual and asexual lineages: genome evolution in Bacillus stick insects

“…Every source of new gene combinations, such as those provided by the reversal to sexual reproduction for the facultatively parthenogenetic B. rossius females, or the cross‐over events at heterozygous loci for B. atticus , generate genetic variability to produce a wealth of new clones in these unisexuals. Even the apomictic parthenogens, B. whitei and B. lynceorum , which produce unreduced eggs (Marescalchi & Scali, 2001) do not completely lose meiotic features: not only is the two‐stage cascade of meiosis retained, but also a strict clonality is not realized in the offspring, because of a subtle recombinational leakage which appears to occur during the first prophase, particularly in B. lynceorum, thus keeping the possibility open for the formation of new clones. Furthermore, the production of triploids, as found in B. atticus and B. lynceorum, is evidence that incorporation of the entire genome occurs during fertilization of parthenogens.…”

Parental species and hybrid descendants of Bacillus (Insecta Phasmatodea) show different patterns of highly amplified, colocalized ribosomal and telomeric sequences

“…Moreover, increased chromosome breakages and rearrangements have been observed in all taxa embodying the B. rossius genome, particularly in the parthenogenetic hybrids ( Figure 5(b,c,d)). In this connection, several NOR features could be passed from B. whitei to B. lynceorum at the time of B. atticus backcrosses to the former parthenogen (Marescalchi & Scali 2001). The LCB and PED females of B. lynceorum are fully in line with this issue for fissions, deletions and new chromosome/fragments appearance.…”

Parthenogenesis and developmental constraints

“…Recently it was found that substantial horizontal transfer of genes has provided genetic diversification in bdelloid and also in ancient monogodont rotifers (hence, no scandals; Castagnone-Sereno, Danchin, Perfus- Barbeoch, & Abad, 2013;Gladyshev, Meselson, & Arkhipova, 2008). Other examples are found in cladocerans, aphids, mites, insects, and the squamate reptiles in vertebrates (Darevsky, Kupriyanova, Uzzel, & Billet, 1985;Marescalchi & Scali, 2001). Sperm-dependent parthenogenesis is a special type of obligate parthenogenesis in which sperm of a related sexual species is necessary to initiate development, is widespread and has evolved in at least 24 genera belonging to seven phyla, for example, nematodes, pseudoarrhenotokous arthropods, stick insects and unisexual fishes, and amphibians (Beukeboom & Vrijenhoek, 1998;Hubbs & Hubbs, 1932;Lampert & Schartl, 2010;Marescalchi & Scali, 2001).…”

“…Other examples are found in cladocerans, aphids, mites, insects, and the squamate reptiles in vertebrates (Darevsky, Kupriyanova, Uzzel, & Billet, 1985;Marescalchi & Scali, 2001). Sperm-dependent parthenogenesis is a special type of obligate parthenogenesis in which sperm of a related sexual species is necessary to initiate development, is widespread and has evolved in at least 24 genera belonging to seven phyla, for example, nematodes, pseudoarrhenotokous arthropods, stick insects and unisexual fishes, and amphibians (Beukeboom & Vrijenhoek, 1998;Hubbs & Hubbs, 1932;Lampert & Schartl, 2010;Marescalchi & Scali, 2001). Facultative parthenogenesis is that females can reproduce both parthenogenetically and sexually.…”