DNA sequences. This regeneration triggers the formation of B new variants, which
can replace neutralized B, initiating new invasions. This cycle, accomplished by
supernumerary chromosomes, has been observed in Mogi-Guaçu river Prochilodus
lineatus population. B invasion process in this species genome was registered on the
late 70’s and early 80’s. In the 90’s, and nowadays, B-chromosomes frequency is
undergoing by a neutralization phase, which one can be confirmed by this study. In
this way, mendelian inheritance pattern, observed in pisciculture realized cross, can
faithfully depict B-chromosome neutrality condition on curimbatá natural population of
this river. Several studies have been carried out targeting to identify specific DNA
sequences in chromosomes by probe utilization. Lots of probes were elaborated
aiming a seek of some answers to fish B-chromosomes origin and structure.
Nonetheless, so far, only probes got with restriction enzymes are known. In this work,
two probes were elaborated from P. lineatus species genome, PM1B, a
supernumerary chromosome specific probe got by chromosomic microdissection and
PG0B, got from total DNA of the referred species. With the help of fluorescent in situ
hybridization technique (FISH), these two probes, PM1B and PG0B, were used in
chromosomic preparations of Mogi-Guaçu river P. lineatus exemplaries. With PG0B
probe utilization, it was observed a DNA sequence compartment between A- en B-
chromosomes, mainly at centromeric regions. This evidence is consistent with
hypothesis of some structural elements are ordinary for A normal complement
chromosomes and for B-chromosomes for this species. PM1B probe, however, has
found homology only in supernumerary chromosomes. Thus, if such supernumerary
elements had been originated from A-elements, these microchromosomes would
have act on an intensive and independent evolution, determining profound
modifications in their DNA. On the other hand, On the other hand if it cannot discard
the possibility of that these elements are originary of completely strange genetic
material to the species, having been in the introduced one for processes of
interespecific hibridization. It cannot be discarded, however, the hypothesis of that
the identity lack between the chromosomes of the complement and the
supernumerary could also be recurrent of a process of formation "of new", where
genomics’ segments could appear in result of normal physiological processes of the
cells.
Key Words: B-chromosome, B-chromosome evolution, mendelian inheritance,
Prochilodus lineatus, Fish citogenetics.