Maize Genomics and Genetics 2024, Vol.15, No.5, 218-227 http://cropscipublisher.com/index.php/mgg 221 suggesting that these genetic changes play a crucial role in the evolutionary divergence of Zea species. Additionally, the non-uniform mutation rates observed across different species indicate that evolutionary pressures may vary significantly within the genus (Orton et al., 2017). 4Zea Species Divergence and Genetic Adaptation 4.1 Genetic differentiation among Zea species Genetic differentiation among Zea species is a complex process influenced by various evolutionary forces, including gene flow, natural selection, and genetic drift. Studies have shown that divergence in Zea species, such as Zeamays ssp. parviglumis and Zeamays ssp. mexicana, has occurred despite continuous gene flow, suggesting that local adaptation plays a significant role in maintaining species boundaries. The identification of key genes involved in species differentiation has been facilitated by genome-wide scans, which have revealed significant SNP associations with environmental variables like temperature and soil phosphorus concentration (Aguirre-Liguori et al., 2019). These findings indicate that specific genomic regions, possibly including putative inversions, contribute to reduced gene flow and increased genetic differentiation between locally adapted populations. In addition to local adaptation, historical gene flow has also played a crucial role in the divergence of Zea species. For instance, sequence polymorphism data from 26 nuclear loci have provided evidence for adaptive and purifying selection at nonsynonymous sites, highlighting the role of gene flow in the evolutionary history of Zea mays ssp. mays and three wild Zea taxa. This study estimated divergence times and suggested rapid diversification of lineages within Zea in the last~150 000 years, further emphasizing the importance of gene flow in shaping genetic differentiation (Ross-Ibarra et al., 2009). 4.2 Natural selection and adaptive traits Natural selection is a driving force in shaping adaptive traits in Zea species. The role of natural selection in adaptive divergence is evident from studies on other taxa, such as Acrossocheilus, where positive selection on mitochondrial genes has been linked to adaptation to different habitats (Zhao et al., 2022). Similarly, in Zea species, natural selection has likely played a crucial role in the evolution of adaptive traits that confer ecological advantages in specific environments. Case studies of adaptive traits in Zea species have highlighted the importance of environmental factors in shaping genetic variation. For example, the divergence between Zea mays ssp. parviglumis and Zea mays ssp. mexicana has been associated with adaptation to temperature and soil phosphorus concentration. Genome-wide scans have identified outlier SNPs linked to these environmental variables, suggesting that natural selection has targeted specific genomic regions to drive adaptive divergence (Aguirre-Liguori et al., 2019). Moreover, the study of genetic differentiation in other species, such as Daphnia pulex, has shown that regions of high gene density and recombination are more divergent, indicating that selection on genes related to local adaptation shapes genome-wide patterns of differentiation (Wersebe et al., 2022). These findings underscore the role of natural selection in driving adaptive traits and genetic differentiation in Zea species. 4.3 Hybridization and its evolutionary Impact Hybridization has played a significant role in the evolution of Zea species, contributing to genetic diversity and adaptive potential. Gene flow between hybridizing taxa can lead to heterogeneous genomic divergence, as observed in the teosinte subspecies Zeamays ssp. parviglumis and Zeamays ssp. mexicana. The continuous gene flow and secondary contact between these subspecies have resulted in genomic regions of high differentiation, likely driven by adaptive divergence and reduced gene flow in locally adapted populations (Aguirre-Liguori et al., 2019). Historical gene flow has also been a key factor in the divergence of Zea species. For instance, cultivated maize (Zea mays ssp. mays) may serve as a bridge for gene flow among otherwise allopatric wild taxa, facilitating the exchange of genetic material and promoting genetic diversity. This historical gene flow has likely contributed to
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