Heterosis is a phenomenon where the offspring of genetically distinct populations exhibit superior vitality, reproductive capacity, and adaptability compared with the average of their parent populations, which is an important genetic resource. Heterosis plays a significant role in modern agriculture, contributing to increase yields and quality of livestock and crops, rapidly improve traits, accelerate the breeding of new varieties, and enhance genetic diversity, thereby efficiently boosting the production of animal husbandry and agriculture while reducing costs. Despite the discovery of heterosis is over a century ago, the elucidation of its genetic basis lags far behind its application in agricultural production. The study of the complex formation mechanism of heterosis is a classic and an active topic in the field of genetics and breeding, but the clear conclusions remain limited. In response to the characteristics of heterosis, scientists have successively proposed various hypotheses for its formation, such as the dominance hypothesis, overdominance hypothesis, and epistasis hypothesis, revealing that the genetic basis of heterosis was non-additive genetic effects. However, these hypotheses are based on the effects of single genes, which are overly idealized and simplistic. Explorations at different levels, such as DNA, RNA, and proteins, have successively discovered the coexistence of multiple genetic effects. Particularly in hybrid crops like rice and corn, the related researches have been continuously identified the loci of heterosis effects, enriched the understanding of the formation mechanism for heterosis in crops, and promoted the transformation of crop breeding technologies, such as precise molecular design breeding. Heterosis is also widely applied in the breeding of livestock and poultry. In developed countries with advanced animal husbandry, over 80% of commercial pork, chicken, and eggs are obtained from hybrid breeds. To efficiently apply heterosis in production for animal husbandry, it is necessary to predict heterosis in advance. New methods, such as the inter- and intra-group phenotypic variance ratio prediction, hybrid heritability prediction, and molecular marker prediction, have been developed to solve the long experimental cycle, environmental sensitivity, and high human and financial costs associated with traditional hybridization experiments for predicting heterosis. However, the accuracy of these prediction methods is limited. Heterosis involves in interaction of multiple levels, and because of the complex genetic background and long breeding cycle, it is still a big challenge for the study of the heterosis formation mechanism and accurate prediction methods. In recent years, the gradual application of sequencing technology has provided a new perspective for understanding the molecular regulatory network of heterosis in livestock and poultry. QTL mapping and genome-wide association study reveal the molecular mechanism of heterosis at the genomic level, and the identified molecular makers are applied in selection and breeding. Combined with multi-omics researches, such as transcriptomics and metabolomics, the key functional genes, variations, and metabolites affecting heterosis can be more precisely located, which facilitate hybrid improvement. This review elaborated the research progress in the formation mechanism and prediction methods for heterosis in the field of livestock and poultry. For looking forward to future, the researches will gradually clarify the complex mechanism of heterosis by integrating multi-omics sequencing data and bioinformatics analysis, in order to identify genes and molecular markers related to heterosis, and innovate new prediction methods, which will provide a more accurate direction for the utilization of heterosis.

Sexual maturation heterosis has been widely exploited in animal crossbreeding. However, the underlying mechanism has been rarely explored in chicken. In the present study, we performed the reciprocal crossing between White Leghorn and Beijing You chicken to evaluate the phenotypes related to sexual maturation, and profiled the ovary circRNAs of purebreds (WW, YY) and crossbreds (WY, YW) to elucidate the molecular mechanism underlying heterosis for sexual maturation. Pubic space and oviduct length exhibited positive heterosis, and age at first egg (AFE) exhibited negative heterosis in the crossbreds. We identified 3,025 known circRNAs and 624 putative circRNAs, which were mainly derived from the exons. Among these circRNAs, 141 and 178 circRNAs were specially expressed in WY and YW, respectively. There were 52.38 and 64.63% of total circRNAs in WY and YW exhibited non-additive expression pattern, respectively. GO enrichment and KEGG pathway analysis showed that the host genes of non-additive circRNAs were mainly involved in TGF-beta signaling pathway, oocyte development, ATPase activator activity, oocyte meiosis, progesterone-mediated oocyte maturation and GnRH signaling pathway. Weighted gene co-expression network analysis identified that 4 modules were significantly (P<0.05) correlated with oviduct length and pubic space. The host genes of non-additive circRNAs harbored in the 4 modules were associated with MAPK signaling pathway and Wnt signaling pathway. Furthermore, competing endogenous RNAs (ceRNA) network analysis characterized non-additive circRNAs gal-FGFR2_0005 and gal-MAPKAP1_0004 could interact with gga-miR-1612 and gga-miR-12235-5p to regulate CNOT6, COL8A1, and FHL2, which were essential for ovary development, indicating that the non-additive circRNAs involved in the formation of sexual maturation heterosis through regulating genes related to the reproductive and developmental process. The findings would provide a deeper understanding of the molecular mechanism underlying sexual maturation heterosis from a novel perspective.