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GENOMIC EVOLUTION IN BACILLUS STICK INSECTS 267

(E) diploid hybrid diploid hybrid diploid hybrid
¥ j genome genome genome
A
DNA \ degeneration of the
DNA X extradoubling ^yivhole hybrid genome
extradoubling

2n tetrachromatidic 2n tetrachromatidic
chromosomes chromosomes

2n autobivalents 2n autobivalents

exclusion of the mixis of two
grandit genome sperm heads

Downloaded by [31.185.101.115] at 11:07 10 April 2016 oocyte with only
rossius autobivalents

production of rossius
pronucleus and polar body

amphimixis

hybrid clonal reconstituted grand» non hybrid
zygoid hybrid zygote male grand» zygote
pronucleus

Fig. 3 - Schematic drawing of cytological mechanisms of Bacillus stick insects. A, apomictic parthenogenesis of B. whitei and B. lynceo-
rum; B, hybridogenesis in B. rossius-grandii hemiclonals; C, androgenesis of hybridogens and B. whitei. Note that androgenetic proge-
ny have nuclear grandii genomes but rossius mitochondrial DNA, since egg cytoplasm ultimately derives from the B. rossius ancestor.
Large metacentrics represent B. grandii; small metacentrics, 'Paleo' B. rossius, and acrocentrics, 'Neo' B. rossius.

gene were started. These two additional genomic com- control region can be utilized at different phylogenetic
partments can add to the phylogenetic picture so far de- levels (Simon et al, 1994). Further, for its matrilinear
fined and give more direct information on DNA vari- transmission, it is a basic approach in the studies of hy-
ability and its evolutionary rates. Satellite DNA are brid taxa since it can trace the maternal ancestor. In ver-
repetitive sequences that represent a substantial part of tebrates, such analyses mainly revealed that most unisex-
the eukaryotic genome. They are typically organized as uals arose through unidirectional hybridization events be-
large (up to 108 bp) tandem array clusters in the hete- tween bisexual species and that, in terms of matriarchal
rochromatic region of chromosomes (e.g., centromeres phylogeny, most unisexual-bisexual complexes show a
and telomeres). The low intraspecific variability of these pattern of paraphyly (Avise et al, 1992). From this kind
tandem arrays, sometimes contrasting with a high level of study it emerged also that some unisexual vertebrate
of interspecific diversity, has been explained through taxa are much more ancient than was expected for 'asex-
several genetic mechanisms leading to a concerted evo- ual' lineages (Spolsky etal, 1992).
lution of highly repeated sequences (Dover & Tautz,
1986; Charlesworth et al, 1994). SatDNA studies have MOST RECENT EXPERIMENTAL RESULTS
been widely used in taxonomic and phylogenetic analy-
ses within many invertebrate groups (Sainz & Cornudel- Satellite DNA analyses
la, 1990; Badaracco et al, 1991; Bachmann & Sperlich,
1993; Rovira et al, 1993; La Volpe, 1994; Bachmann et Molecular investigations on Bacillus genomes through
al, 1996; Ugarkovic et al, 1996). canonical genomic restriction and Southern blot hy-
bridization showed the existence of a satDNA family
Recently, mitochondrial DNA has received considerable named Bag320 in B. atticus and B. grandii and its ab-
attention (Wolstenholme, 1992). Its analysis is being suc- sence from B. rossius (Mantovani et ah, 1993, 1997).
cessfully applied in evolutionary studies, since mitochon- Nucleotide sequences obtained from B. atticus and B.
drial protein coding sequences, ribosomal genes, and the