Genomic evolution in parental and hybrid taxa of the genus Bacillus (Insecta Phasmatodea)

Abstract

Fifteen‐year‐long investigations have demonstrated that bisexual Bacillus species (B. rossius and B. grandii), although sharply differentiated for allozyme genes (Nei's D, 1.23–1.62), have hybridized several times in Sicily, generating hybridogenetic strains (B. rossius‐grandii) and parthenogenetic taxa, the diploid B. whitei (B. ros‐sius/B. grandii) and, together with the telytokous taxon B. atticus, the triploid trihybrid B. lynceorum (B. rossius/B. grandii/B. atticus). B. atticus is much closer to B. grandii (D, 0.31–0.36) than to B. rossius (D = 1.82). Furthermore, SEM and allozymatic investigations allowed it to be established that body and egg characters are more conserved than gene‐enzyme systems. Recently, satellite DNA sequences (the Bag320 family) and a mitochondrial coding gene (COII) were also analysed. It could be noticed that Bag sequences provided differentiation values among taxa higher than coding nuclear genes. Furthermore, it was discovered that, while parthenogenesis in B. atticus leaves random levels of individual variability of repeats ‐ likely due to the absence of chromosome shuffling in the progeny ‐, the nucleotide sequences of B. grandii split into subspecific clusters, indicating that sequence variant homogeniza‐tion/fixation are at work in Mendelian reproducing species. In B. whitei and B, lynceorum, only a limited sequence variability of grandii‐like repeats was observed, that suggested a rather recent origin for them, although sufficiently old to allow gene‐conversion to start between grandii‐like and atticus‐like sequences in B. lynceorum. The invention of both grandii‐like and atticus‐like Bag 320 clones also established beyond doubt the B. atticus contibu‐tion to the structure of this triploid hybrid. MtDNA of the COII gene gave indications of a more homogeneous degree of differentiation among taxa than that suggested by nuclear compartments, since B. rossius, B. grandii, and B. atticus appear to be differentiated from each other to a similar degree. Furthermore, COII unexpectedly demonstrated that, in addition to hybridogens and B. whitei, also B. lynceorum has B. rossius as the maternal ancestor, thus indicating a hybridization route different from that previously thought for this hybrid. Finally, mtDNA served as a genetic marker to demonstrate that androgenesis does occur in nature, since from hybridogenetic females, B. grandii specimens were produced, carrying mtDNA of B. rossius. Androgens provide an unusual opportunity of genome evolution since gene‐size fragments of DNA can escape from mitochondria and migrate to the nucleus.

Key words:

• Stick insects
• Genome evolution
• mtDNA ‐satDNA
• Allozymes
• Hybrid parentage
• Parthenogenesis
• Natural androgenesis.