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For popcorn, obtaining and identifying haploids are still challenging steps. We aimed to induce and screen haploids in popcorn using the Navajo phenotype, seedling vigor, and ploidy level. We used the Krasnodar Haploid Inducer (KHI) in crosses with 20 popcorn source germplasms and five maize controls. The field trial design was completely randomized, with three replications. We assessed the efficacy of induction and identification of haploids based on the haploidy induction rate (HIR) and false positive and negative rates (FPR and FNR). Additionally, we also measured the penetrance of the Navajo marker gene (R1-nj). All putative haploids classified by the R1-nj were germinated together with a diploid sample and evaluated for false positives and negatives based on vigor. Seedlings from 14 females were submitted to flow cytometry to determine the ploidy level. The HIR and penetrance were analyzed by fitting a generalized linear model with a logit link function. The HIR of the KHI, adjusted by cytometry, ranged from 0.0 to 1.2%, with a mean of 0.34%. The average FPR from screening based on the Navajo phenotype was 26.2% and 76.4% for vigor and ploidy, respectively. The FNR was zero. The penetrance of R1-nj ranged from 30.8 to 98.6%. The average number of seeds per ear in temperate germplasm (76) was lower than that obtained in tropical germplasm (98). There is an induction of haploids in germplasm of tropical and temperate origin. We recommend the selection of haploids associated with the Navajo phenotype with a direct method of confirming the ploidy level, such as flow cytometry. We also show that haploid screening based on Navajo phenotype and seedling vigor reduces misclassification. The origin and genetic background of the source germplasm influence the R1-nj penetrance. Because the known inducers are maize, developing doubled haploid technology for popcorn hybrid breeding requires overcoming unilateral cross-incompatibility.
For popcorn, obtaining and identifying haploids are still challenging steps. We aimed to induce and screen haploids in popcorn using the Navajo phenotype, seedling vigor and ploidy level. We used the Krasnodar Haploid Inducer (KHI) in crosses with 20 popcorn source germplasms and five maize controls. The field trial design was completely randomized, with three replications. We assessed the efficacy of induction and of identification of haploids based on haploidy induction rate (HIR) and false positive and negative rates (FPR and FNR). Additionally, we also measured the penetrance of the Navajo marker gene (R1-nj). All putative haploids classified by R1-nj were germinated together with a diploid sample and evaluated for false positives and negatives based on vigor. Seedlings from 14 females were submitted to flow cytometry to determine the ploidy level. The HIR and penetrance were analyzed by fitting a generalized linear model with a logit link function. The HIR of the KHI, adjusted by cytometry, ranged from 0.0 to 1.2%, with a mean of 0.34%. The average FPR from screening based on the Navajo phenotype were 26.2% and 76.4%, by the vigor and ploidy, respectively. The FNR was zero. The penetrance of R1-nj ranged from 30.8 to 98.6%. The average number of seeds per ear in temperate germplasm (76) was lower than that obtained in tropical germplasm (98). There is induction of haploids in germplasm of tropical and temperate origin. We recommend the selection of haploids associating the Navajo phenotype with a direct method of confirming the ploidy level, such as flow cytometry. We also show that haploid screening based on Navajo phenotype and seedling vigor reduces misclassification. The origin and genetic background of the source germplasm influence the R1-nj penetrance. Because the known inducers are maize, developing doubled-haploid technology for popcorn hybrid breeding requires overcoming the unilateral cross-incompatibility.
Cytogenomics in Zea mays L. have been provided physical maps, which gather data from cytogenetics, genetic maps and genome sequencing. Since McClintock's data about the basic chromosome number (x = 10), Z. mays karyotype has been characterized, evidencing the dynamism of its genome. Haploid induction is the initial step to produce double haploid lines, which are genetically homozygous for all loci in up to two generations. Haploid induction is a quantitative trait related to different genes, such as r-navajo (r1), colored plant1 (b1), matrilineal1 (mtl) and membrane protein domain1 (dmp1). The physical loci of these genes are still unknown. We aimed to map the physical loci of the r1, b1, mtl and dmp1 genes related to Z. mays haploid induction. For this, seeds of Z. mays ‘Krasnodar’ haploid inducer and of different genitors were used. We co-hybridizated probes for rDNA 18S + r1, 18S + b1, 18S + mtl, and 18S + dmp1. Two hybridization signals for each gene were identified in five interphase nuclei for all lines. The mapping of these sequences in Z. mays metaphase chromosomes evidenced specific locus for each gene. 18S rDNA is located on chromosome 6 short arm, mtl on chromosome 1 short arm, b1 on chromosome 2 short arm, dmp1 on chromosome 9 short arm, and r1 on chromosome 10 long arm. Due to specific locus, these genes can also be used as chromosome- specific cytogenetic markers for Z. mays and the 18S rDNA. We conclude that the construction of physical maps contributes to understanding the structure and evolution of Z. mays genome, increasing the cytogenomic data about this species. Keywords: Cytogenetics. Double haploid. FISH. Plant breeding. Maize.
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