American Criollo pigs are thought to descend mainly from those imported from the Iberian Peninsula starting in the late 15th century. Criollo pigs subsequently expanded throughout the Americas, adapting to very diverse environments, and possibly receiving influences from other origins. With the intensification of agriculture in the mid-20th century, cosmopolitan breeds largely replaced Criollo pigs, and the few remaining are mostly maintained by rural communities in marginal areas where they still play an important socio-economic and cultural role. In this study, we used 24 microsatellite markers in samples from 1715 pigs representing 46 breeds with worldwide distribution, including 17 American Criollo breeds, with the major focus of investigating their genetic diversity, structure and breed relationships. We also included representatives of the Iberian, Local British, Hungarian, Chinese and Commercial breeds, as well as Wild Boar, in order to investigate their possible influence in the genetic composition of Criollos. Our results show that, when compared with the other breeds, Criollo pigs present higher levels of genetic diversity, both in terms of allelic diversity and expected heterozygosity. The various analyses indicate that breed differentiation overall explains nearly 21% of the total genetic diversity. Criollo breeds showed their own identity and shared a common genetic background, tending to cluster together in various analyses, even though they differ from each other. A close relationship of Criollos with Iberian breeds was revealed by all the different analyses, and the contribution of Iberian breeds, particularly of the Celtic breeds, is still present in various Criollo breeds. No influence of Chinese breeds was detected on Criollos, but a few were influenced by Commercial breeds or by wild pigs. Our results confirm the uniqueness of American Criollo pigs and the role that Iberian breeds have played in their development.

An mRNA expression study has been performed in 20- to 25-min postmortem-obtained samples from Longissimus dorsi muscle of 59 Duroc × Landrace/ Large White (LD/LW) pigs to search for gene sequences related to meat quality (pH(24), pH(45), "Lab" color coordinates, curing yield, and exudation at 3 different times) or meat composition [intramuscular fat, content of several fatty acids (C16:0, C18:0, C18:1, and C18:2), ratio of saturated, monounsaturated, and polyunsaturated fatty acids, and protein and humidity contents] traits to find targets for selection. Gene ontology analysis, biological pathway, and gene network studies all show that many more differentially expressed genes (506 vs. 279) are related to meat quality (Group P or perimortem characters) than to meat composition traits (Group L or whole life traits). The difference between the number of gene ontology terms annotated, biological pathways, and gene networks in groups P and L is notable due to the differences in the complexity of the "generation process" of P-traits and involvement of other tissues or organs in the generation of variability of L-traits. Also, interactions between differentially expressed genes were found in extracellular matrix (ECM)-receptor interaction, tumor growth factor (TGF)-beta signaling pathway, fatty acid elongation in mitochondria, and adipocytokine signaling pathway, indicating that a substantial fraction of the gene networks could be associated with interactions between differentially expressed genes related to traits under study. A large number of the most overexpressed genes are related to muscle development and functionality and repair mechanisms; they could be good candidates for breeding programs in which the main goal is to enhance meat quality.