Bioscience Evidence 2024, Vol.14, No.5, 195-205 http://bioscipublisher.com/index.php/be 196 limited genetic diversity within the wheat gene pool poses a bottleneck for breeding programs. Utilizing genetic resources from wild relatives and other Triticeae species can help overcome this limitation and introduce new alleles for stress resistance and yield improvement (Mondal et al., 2016; Ali et al., 2020). This study provides a comprehensive overview of the advancements in wheat hybrid breeding and the strategies to overcome biological barriers. It summarizes the current state of wheat hybridization techniques and their application in breeding programs, evaluates the potential of hybrid wheat in improving yield and stress resistance, identifies the key biological barriers in hybrid wheat development, and proposes innovative approaches to address these challenges. Furthermore, it highlights the future prospects and research directions for hybrid wheat breeding. By addressing these objectives, the study contributes to the ongoing efforts to tackle current challenges, enhance wheat production, and ensure global food security. 2 Biological Obstacles Challenges in Wheat Hybridization 2.1 Reproductive barriers Reproductive barriers are significant challenges in wheat hybridization, often manifesting as incompatibility between different wheat species and sterility issues in hybrids. These barriers can be prezygotic, such as pollen-pistil incompatibility, or postzygotic, including hybrid sterility and inviability. For instance, hybrid sterility is a common issue when crossing hexaploid bread wheat (Triticum aestivum) with tetraploid durum wheat (Triticum turgidum), leading to pentaploid hybrids that frequently exhibit sterility and poor seed set (Padmanaban et al., 2017). This sterility is often due to genetic incompatibilities that disrupt normal meiotic processes, resulting in unviable gametes (Forsdyke, 2018). Moreover, hybrid sterility can be exacerbated by epistatic interactions between divergent loci from the parent species. These interactions can lead to dysfunctional gene networks, further reducing hybrid fertility. For example, in the case of Solanum species, it was found that hybrid sterility often results from complex genetic interactions rather than simple pairwise incompatibilities, indicating a similar potential mechanism in wheat hybrids (Guerrero et al., 2016). Overcoming these reproductive barriers requires careful selection of parental genotypes and advanced breeding techniques to ensure compatibility and fertility in the resulting hybrids. 2.2 Chromosomal differences Chromosomal differences pose another significant challenge in wheat hybridization, particularly due to the difficulty in successful chromosome pairing between species with different ploidy levels. Hexaploid wheat (Triticum aestivum, 2n=6x=42) and tetraploid wheat (Triticum turgidum, 2n=4x=28) have different numbers of chromosome sets, complicating meiosis and leading to issues such as unbalanced gametes and reduced fertility (Padmanaban et al., 2017). The presence of homeologous chromosomes, which are similar but not identical, further complicates pairing during meiosis, often resulting in meiotic instability and chromosomal rearrangements (Parisod and Badaeva, 2020). Interspecific hybridization can lead to significant chromosomal restructuring, as observed in wild wheat species. These structural changes can include chromosomal fusions, fissions, and translocations, which can disrupt normal genetic function and lead to hybrid sterility or inviability (Parisod and Badaeva, 2020). Additionally, the phenomenon of "genome shock" in synthetic allotetraploid wheat, where newly formed hybrids exhibit extensive karyotype variation and meiotic irregularities, highlights the challenges posed by chromosomal differences in hybridization efforts (Sha et al., 2023). Addressing these challenges requires advanced cytogenetic techniques and a deep understanding of the genetic and chromosomal architecture of the parent species. 2.3 Hybrid seed production challenges Hybrid seed production in wheat is often hindered by the limitations of male sterility systems, which are crucial for preventing self-pollination and ensuring cross-pollination. One of the primary bottlenecks in hybrid wheat breeding is the lack of an efficient and practical sterility system. Traditional male sterility systems, such as cytoplasmic male sterility (CMS), have limitations, including incomplete sterility and the need for fertility restorers, which complicate breeding programs (Kempe et al., 2014).
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