This study aims at identifying mammary gland genes expressed in Brazilian Holstein cattle produced under tropical conditions, as compared to the Portuguese Holstein cattle produced in a temperate region. For this purpose, cDNA microarrays and real-time (RT) PCR transcriptomic techniques were utilized in 12 Holstein cows from the same lactating phase and management systems divided into two groups: Holstein Brazil (HB) originated from Brazil and Holstein Portugal (HP) from Portugal. The genomic results show that from a total of 4608 genes available from the microarray slide (Bovine Long Oligo (BLO) library), 65 transcripts were identified as differentially expressed in mammary glands. The genes associated with mammary gland development and heat stress responses showed greater expression in HB animals. In the HP group, upregulated genes related with apoptosis and vascular development and downregulated genes related with resistance to heat stress were observed. Validation of microarray results was done using RT-PCR. HB animals had higher blood levels of growth hormone than HP animals. Blood levels of prolactin and T were similar for both groups and GH levels were increased in the HB group. The results suggest a gene change towards long-term acclimatization of Brazilian Holstein cattle to cope with tropical heat stress conditions.

This study aims at examining the expression of genes on the mammary gland, corresponding to various levels of adaptation or acclimatization to environmental stress. The authors utilized 18 cows from three genetic groups, Holstein Brazil (HB), Gyr (GG) and Gyrolando (GH ), all in the same stage of lactation, and subjected them to the same management conditions. Venous and arterial blood were collected to determine the hormonal profile and blood chemistry. Mammary gland tissue was used for transcriptomic studies. Prolactin and GH plasmatic concentrations were higher in Holstein animals. There were no differences in IGF-1 concentrations among the experimental groups. T 3 concentrations were similar among the Holstein and Gyr groups. From the 4608 transcripts in the BLO-Bovine EST (Michigan State University, US) databank that were used in this experiment, 105 differentially expressed genes were identified in at least one of the groups. Among these, the authors highlighted 14 genes that were related to the structure of the mammary gland (CRDGF, CD97, GH, endoglin, LTF, INPP, PTP), to response to thermal stress (Crh_11, v-Fos, Cdc37) and to milk protein (RPL35, κ-casein, β-casein, α-s2-casein). Eight of these were validated through real-time polymerase chain reaction. The HB animals, in comparison with the GH and Gyr groups, presented up-regulated genes associated with epithelium cellular differentiation and proliferation, milk productivity and decreased heat stress tolerance. Gyr animals presented up-regulated transcripts associated with cellular defence, apoptosis processes and increased tolerance to heat stress. The GH group showed intermediary results compared with the other two groups. ______________________________________________________________________________________