Introduction

Genetic recombination is a fundamental biological process involving the exchange of DNA segments between homologous chromatids during meiotic prophase I. This process includes both crossover and non-crossover events, generating new allelic combinations and increasing genomic variability within species. Alongside spontaneous DNA mutations, meiotic recombination acts as a major evolutionary force that drives genome diversification, adaptation, and species evolution. In crop plants such as Maize, recombination plays a critical role in plant breeding by reshuffling polymorphic loci, enabling the selection of favorable allele combinations while reducing the accumulation of deleterious mutations.

Recombination frequency is commonly quantified as the ratio between genetic distance and physical genome distance, expressed in centimorgans per megabase (cM/Mb). This parameter can be evaluated at both the genome-wide scale, referred to as the genome-wide recombination ratio (GWRR), and at specific chromosomal loci, known as the locus recombination ratio (LRR). Recombination events are not evenly distributed across chromosomes. Instead, distinct recombination hotspots frequently occur in genomic regions exhibiting elevated recombination activity.

In maize, several recombination hotspot regions have been identified near important genetic loci such as the Bronze locus and the anthocyanin1 gene. These regions are often associated with gene-rich chromosomal segments. Recombination frequency is influenced not only by local genomic architecture but also by specific meiotic genes involved in homologous chromosome pairing and DNA repair mechanisms, including PHS1, RAD50, and RAD51. These findings demonstrate that recombination is under strong genetic regulation.

Recent advances in ultra high density genotyping technologies have significantly improved the ability to investigate recombination landscapes at the whole-genome level. Previous studies using genotyping by sequencing and SNP arrays revealed that recombination patterns in maize are relatively stable across some populations but may vary considerably among different genetic backgrounds. Such variation suggests the presence of intraspecific recombination diversity within maize germplasm.

Despite these advances, several key biological questions remained unresolved. In particular, the relative contribution of intragenic recombination (within genes) versus intergenic recombination (between genes) to overall recombination variability and phenotypic diversity was poorly understood. Earlier research indicated that recombination events frequently occur in gene-associated regions, especially near untranslated regions (UTRs) and regulatory elements. Because recombination is strongly enriched within genes, understanding how these events influence gene expression and agronomic traits became an important objective in maize genomics research.

To address these questions, researchers constructed high-resolution linkage maps from 12 segregating maize populations using the Illumina MaizeSNP50 genotyping platform. These populations originated from crosses among diverse maize inbred lines and one teosinte accession. The study identified both conserved recombination hotspots shared across populations and population-specific hotspots unique to particular genetic backgrounds. Recombination events were further classified into intragenic and intergenic categories to investigate their contribution to genomic variation.

Mapping Populations and Genotyping Strategy

The study analyzed 12 independent maize mapping populations, including recombinant inbred lines (RILs) and one BC2F5 backcross population. These populations were derived from crosses involving 16 maize inbred lines and one teosinte genotype. Several parental lines originated from globally diverse germplasm collections, including landrace-derived inbred lines from Peru and the United States.

More than 2,400 segregating lines were genotyped using the Illumina MaizeSNP50 BeadChip, containing over 56,000 single nucleotide polymorphism (SNP) markers. Following stringent quality-control filtering based on missing genotype frequency and heterozygosity thresholds, approximately 2,319 high-quality lines were retained for downstream analyses.

High-density linkage maps were generated using integrated bioinformatics pipelines combining CARTHAGENE software and custom PERL scripts. The resulting maps contained an average of more than 13,000 polymorphic markers per population with highly refined marker spacing, enabling accurate detection of recombination breakpoints across the maize genome.

Detection and Characterization of Recombination Events

Recombination events were estimated by analyzing genotype transitions between adjacent SNP markers. Missing genotype information was imputed using neighboring marker patterns and genetic distances. Changes in parental haplotypes between flanking markers were interpreted as recombination breakpoints.

Genome-wide recombination landscapes were constructed using sliding-window analyses with 2-Mb genomic intervals. Recombination activity varied substantially across chromosomes. Pericentromeric regions generally exhibited low recombination rates, whereas telomeric and gene-rich chromosomal regions displayed significantly elevated recombination frequencies.

The analysis identified 143 recombination hotspot regions distributed across 26 distinct genomic intervals. Some hotspots were consistently detected across multiple populations, indicating conserved recombination mechanisms, whereas others were highly population-specific, reflecting the influence of genetic background on recombination dynamics.

Intragenic and Intergenic Recombination Patterns

To better understand recombination diversity, SNP markers were categorized as either genic or intergenic based on their physical positions relative to annotated maize genes. Approximately 70% of segregating SNPs were located within genes, enabling detailed characterization of intragenic recombination.

More than 3,000 genes exhibited detectable intragenic recombination events in at least one population. Interestingly, many recombination-associated genes were shared among multiple populations, suggesting partial conservation of recombination-prone genomic regions.

A notable discovery was the contrasting behavior of intragenic and intergenic recombination. The total number of intragenic recombination events remained relatively stable across different maize populations, whereas intergenic recombination varied substantially depending on genetic background. Statistical analyses demonstrated that intergenic recombination contributed more strongly to overall recombination variability among populations.

This observation may be explained by structural genomic variation among maize inbred lines. Genes tend to be highly conserved across different genomes, whereas intergenic regions contain large numbers of transposable elements and structural rearrangements. Consequently, recombination within genes remains comparatively stable, while intergenic recombination fluctuates according to genomic structural diversity.

Relationship Between Recombination, Gene Expression, and Phenotypic Variation

The study further investigated whether recombination events influence gene expression and agronomic traits. Using haplotype-based association mapping in a panel of over 500 maize inbred lines, researchers identified significant associations between intragenic recombination events and multiple phenotypic traits.

Genes undergoing intragenic recombination were more likely to exhibit elevated expression levels across different tissues and developmental stages. These genes also showed stronger enrichment for cis-expression quantitative trait loci (cis-eQTLs), suggesting that recombination contributes to local regulatory variation controlling transcriptional activity.

Association analyses identified significant relationships between recombination-derived haplotypes and several important agronomic traits, including:

• Plant height
• Ear height
• Kernel row number
• Kernel size and weight
• Flowering time
• Oil composition
• Cob color

One particularly important example involved the gene GRMZM2G115615, which was significantly associated with total kernel oil concentration. Recombination-generated haplotypes within this gene produced measurable phenotypic differences in oil content, demonstrating how recombination can generate novel functional alleles with agronomic importance.

Biological and Breeding Implications

This study provides a comprehensive view of the recombination landscape in maize and highlights the biological importance of both intragenic and intergenic recombination processes. The identification of conserved and population-specific recombination hotspots offers valuable insights for modern plant breeding programs.

Understanding recombination dynamics can improve marker-assisted selection strategies by enabling breeders to target genomic regions with elevated recombination frequencies. This may accelerate the generation of favorable allele combinations and enhance genetic gain during crop improvement.

The findings also demonstrate that intragenic recombination contributes directly to gene-expression variability and phenotypic diversity. These mechanisms likely played a major role during maize domestication and continue to influence modern breeding outcomes.

Conclusion

Genome-wide recombination dynamics represent a major source of genetic and phenotypic diversity in maize. High-density linkage mapping revealed extensive variability in recombination frequency across populations and genomic regions. While intergenic recombination appears to drive much of the recombination variation observed among different genetic backgrounds, intragenic recombination strongly influences gene regulation and agronomic trait expression.

These discoveries significantly improve our understanding of recombination biology in maize and provide important genomic resources for future crop breeding and functional genomics research.