Mapeamento de QTL para resistência a parasitas e características de crescimento nos cromossomos cinco e sete de uma população experimental F2 de bovinos Gir x Holandês
Resumo
The large amount of phenotypic variability between Bos primigenius taurus and Bos
primigenius indicus cattle, which had diverged hundreds of thousands of years ago and since
then have been explored by men through artificial selection of the breeds to milk and meat
production, allows the development of crossbreeds, since there is no reproductive isolation
between these subspecies. It is known that part of the described differences for productive and
parasite resistance traits among the breeds is under genetic control, and that is possible to
distinguish genetically superior individuals from the others of the population with the
combined use of molecular genetics and traditional livestock production programs. Since in
Brazil the economy is largely influenced by agribusiness, once bovine meat exportation alone
generated US$2.5 bilion in 2004, the search and identification of chromosomic regions that
control al least part of the variation of these quantitative economic traits (QTL, Quantitative
Trait Loci), like birth weight and weight gain, and also regions that influence resistance to
parasites, is of great concern. The objective of the present study was to map QTLs for growth
traits and resistance to gastrointestinal endoparasites and external parasites, like the tick
(Riphicephalus (Boophilus) microplus) and the beef worm (Dermatobia hominis) using
microsatellite markers to scan bovine chromosomes five and seven in a F2 Gir x Holstein
experimental population. On chromosome five, a significant (P < 0.01) QTL for birth weight
and one suggestive (P < 0.05) for resistance to tick count during the rainy season were
detected. On chromosome seven, it was identified an indicative QTL (P < 0.10) for tick count
in the dry season. In both cases, a significant dominance deviation was observed, suggesting
that part of the genetic variation must be atributed to allele combinations. The favorable allele
for resistance in chromosome five was from the Gir parental and on chromosome seven, from
Holstein, which is a surprising result, since the zebu breeds were traditionally considered as
the solely source of tick resistance. No QTL was associated to resistance to endoparasites or
to the beef worm. The application of these informations still demands the reduction of
confidence intervals for the QTLs locations and validation in other populations.