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270 B. MANTOVANI, M. PASSAMONTI, V. SCALI

hypothesized that a directional mutation pressure to- the other hand, a mitochondrial B. rossius genome was

wards a high A+T content has been realized not only in quite unexpected for B. lynceorum, since the most like-

the endopterygote phylads, but, episodically, also in ly maternal ancestor was assumed to be the nowadays

some exopterygote lineages such as the Phasmatodea. strictly unisexual B. atticus. Discovering instead that B,

Further, we would like to point out the peculiar find- rossius was involved in the first hybridization step, we

ing of two Bacillus specimens with a B. grandii grandii would have to admit that at the time of B. lynceorum

nuclear genome —as parallel allozyme and satDNA origin, B. atticus was a bisexual taxon. A peculiar route

analyses showed — which carried mtDNA of B. rossius. has also been suggested for the origin of some unisexu-

The finding verifies that androgenesis can occur in the al triploid Ambystoma (Kraus & Miyamoto, 1990). Situa-

field as previously hypothesized (Mantovani & Scali, tions as that of B. lynceorum and Ambystoma unisexu-

1992; Tinti & Scali, 1996). Androgenesis was evidenced als clearly evidence that to trace the true series of hy-

in the Bacillus hybrid strains of B. rossius-grandii bridization steps in a reticulate evolution, requires a

through a series of allozymic, chromosomal and cyto- punctual demonstration of the various inherited

logical investigations (Mantovani & Scali, 1992; Tinti & genomes; this has been attempted with some success

Scali, 1993, 1995, 1996). These females, found in two for Bacillus taxa.

Downloaded by [31.185.101.115] at 11:07 10 April 2016 isolated Sicilian areas (corners of north-western and As a by-product of mtDNA analysis, it could be
south-eastern Sicily; Fig. 1) derive from B. rossius x B. shown that unisexual Bacillus arose through asymmetri-
grandii crosses; their reduced eggs pass the whole cal hybridization events with only one cross direction of
unassorted B. rossius haploset to the next generation the two possible being realized in nature. As occurs in
(hemiclonal transmission). Hybrids are produced anew most extant hybrid vertebrates (Avise et al, 1992),
each generation when such eggs are fertilized by syn- Bacillus hybrids derive from a subset of the matriarchal

topic B. grandii males. The previously discarded pater- genealogy of the sexual parent (paraphyly).

nal set of chromosomes is thus replaced and the hybri-

dogenetic system - actually the only one known among

invertebrates - maintained. Among the lab-recorded B.
rossius-grandii hybridogenetic offspring all-paternal, CONCLUDING REMARKS

fully fertile B. grandii specimens of both sexes (andro- If we now try to make use of the above reported
genetics) were produced at a low rate (about 1%). findings in terms of genome evolution, it could be no-
These individuals bring together a nuclear grandii ticed that the different domains we analysed are: nu-
genome to a cytoplasmic rossius mtDNA. In Bacillus, an- clear coding genes, nuclear non-coding sequences, and
drogenetics may sustain the persistence of the fathering a mitochondrial coding gene. We can now compare
taxon, so that, through natural androgenesis, hybridoge- their evolutionary features.
netic females do not act as complete sexual parasites
(see Schultz, 1969; Mantovani & Scali, 1992). Parental First of all, single-copy genes for metabolic enzymes
species and derived hybrids must therefore receive par- appeared to evolve at a rather high rate, while those
ticular attention, since a wide variety of non-canonical re- controlling morphological characters appeared fairly
productive interactions may occur in the field. conserved. This conclusion is based on the observation
that body and egg morphology, at light and scanning

Parentage o/Bacillus hybrids electron microscopy levels, although clearly diagnostic
did not adequately substantiate the very high genetic di-

MtDNA analysis unmistakably demonstrates that B. vergence that electrophoretic results on 20 gene loci

rossius is the maternal ancestor of all Bacillus hybrid solidly supported. Roughly speaking, body morphology

taxa (Mantovani et al., submitted; Fig. 5). Since in all and ootaxonomy would indicate a similar degree of di-

hybridogenetic organisms so far known, the egg keeps vergence among B. rossius, B. grandii, and B. atticus,

the maternal ancestor genome, this was expected for whereas allozymes point to a 5-fold higher differentia-

the hybridogenetic strains because the B. rossius haplo- tion between B. rossius and B. grandii than between B.

set is the hemiclonally transmitted genome. It could be grandii and B. atticus (Scali et al, 1995).

argued that the same should hold true for the diploid B. SatDNA repeats appear to stress further the picture

whitei, since it has the same hybrid constitution as hy- drawn by allozyme species characterization: in B. ros-

bridogens. It must be noted though that the hybridiza- sius such sequences could not be evidenced by canoni-

tion crosses leading to the parthenogen had to be inde- cal restriction methodology, and only PCR amplification

pendent from those which gave rise to hybridogens. Ac- could detect them in some populations (unpublished).

tually, the B. rossius chromosome haplosets are mor- Although such a satDNA family must have been present

phologically distinguishable in the two hybrids: the pa- in a common ancestor of the bisexual taxa, it apparent-

leo- rossius has a second metacentric chromosome and ly underwent sharp copy number variation when

it is found in B. whitei and B. lynceorum, whereas the grandii and rossius lineages split up. It could also be

neo- rossius invariably shows a corresponding acrocen- thought that even the paleo-ross/MS genome, nowadays

tric element which is found in hybridogens and all ex- embodied by parthenogenetic hybrids (B. whitei and B.

tant B. rossius populations (Manaresi et al, 1992). On lynceorum) had a very low copy number of satDNA re-