News Update on Maize genetics : Nov 2021

The Genetics of Maize Evolution

Maize and its closest wild relatives, the teosintes, differ strikingly in the morphology of their female inflorescences or ears. Despite their divergent morphologies, several studies indicate that some varieties of teosinte are cytologically indistinguishable from maize and capable of forming fully fertile hybrids with maize. Molecular analyses identified one form of teosinte (Zea mays ssp. parviglumis) as the progenitor of maize. Analyses of the inheritance of the morphological traits that distinguish maize and teosinte indicates that they are under the control of multiple genes and exhibit quantitative inheritance. Nevertheless, these analyses have also identified a few loci of large effect that appear to represent key innovations during maize domestication. Remaining challenges are to identify additional major and minor effect genes, the polymorphisms within these genes that control the phenotypes, and how the combination of the individual and epistatic effects of these genes transformed teosinte into maize. [1]

MaizeGDB, the community database for maize genetics and genomics

The Maize Genetics and Genomics Database (MaizeGDB) is a central repository for maize sequence, stock, phenotype, genotypic and karyotypic variation, and chromosomal mapping data. In addition, MaizeGDB provides contact information for over 2400 maize cooperative researchers, facilitating interactions between members of the rapidly expanding maize community. MaizeGDB represents the synthesis of all data available previously from ZmDB and from MaizeDB—databases that have been superseded by MaizeGDB. MaizeGDB provides web‐based tools for ordering maize stocks from several organizations including the Maize Genetics Cooperation Stock Center and the North Central Regional Plant Introduction Station (NCRPIS). Sequence searches yield records displayed with embedded links to facilitate ordering cloned sequences from various groups including the Maize Gene Discovery Project and the Clemson University Genomics Institute. An intuitive web interface is implemented to facilitate navigation between related data, and analytical tools are embedded within data displays. Web‐based curation tools for both designated experts and general researchers are currently under development. [2]

Genetics of gene expression surveyed in maize, mouse and man

Treating messenger RNA transcript abundances as quantitative traits and mapping gene expression quantitative trait loci for these traits has been pursued in gene-specific ways. Transcript abundances often serve as a surrogate for classical quantitative traits in that the levels of expression are significantly correlated with the classical traits across members of a segregating population. The correlation structure between transcript abundances and classical traits has been used to identify susceptibility loci for complex diseases such as diabetes1 and allergic asthma2. One study recently completed the first comprehensive dissection of transcriptional regulation in budding yeast3, giving a detailed glimpse of a genome-wide survey of the genetics of gene expression. Unlike classical quantitative traits, which often represent gross clinical measurements that may be far removed from the biological processes giving rise to them, the genetic linkages associated with transcript abundance affords a closer look at cellular biochemical processes. Here we describe comprehensive genetic screens of mouse, plant and human transcriptomes by considering gene expression values as quantitative traits. We identify a gene expression pattern strongly associated with obesity in a murine cross, and observe two distinct obesity subtypes. Furthermore, we find that these obesity subtypes are under the control of different loci. [3]

Application of SSR Markers for Genetic Purity Analysis of Parental Inbred Lines and Some Commercial Hybrid Maize (Zea mays L.)

Aims: Morphological evaluation of seeds and growing plants used for certification for purity and variety distinctness in Nigeria is time consuming and expensive. This experiment set to evaluate the usefulness of SSR markers to determine genetic purity of commercial hybrids and their inbred lines.

Place and Duration of Study: Bioscience unit, International Institute of Tropical Agriculture, Nigeria in December, 2011

Methodology: Seedlings of four F1 hybrids and four inbred lines were grown in the screen house of IITA for DNA extraction using Dellaporta method with some modifications. Six Simple Sequence Repeat (SSR) markers were used for Polymerase Chain Reaction (PCR) using Touch-Down PCR profile. The analysis is by fragment analysis as present (1) or absent (0) Mathematical equation to determine genetic purity of the genotypes was developed from the genetic distances matrix.

Results: Simple descriptive analysis revealed that average genetic diversity and polymorphism information content (PIC) recorded by the markers was 0.592 and 0.512 respectively. Genetic purity level of inbred lines ranged between 91.3% and 98.7% while the hybrids ranged between 81.3% and 95%.

Conclusion: SSR markers are powerful biotechnological tool capable of detecting genetic purity status of Nigerian maize hybrids therefore inclusion of DNA analysis of seeds using SSR markers to determine genetic purity of maize seed is recommended. However, further research work with larger number of seed samples per variety will be needed to validate reliability.[4]

Heritability, Genetic Advance and Correlations in 254 Maize Doubled Haploid Lines × Tester Crosses under Drought Conditions

One of the major advantages of doubled haploid lines (DHL) is the maximum genetic variance between lines for testcross performance from the first generation. Two hundred fifty four testcrosses were produced as a result of crossing between 254 DHL’s and an inbred line tester. The objectives were: (i) to determine the genotypic (GCV) and phenotypic (PCV) coefficients of variation, heritability (h2b) and genetic advance (GA) from selection under water stressed  at flowering (WSF) and grain filling (WSG) and under well-watered (WW) and (ii) to identify the traits of significant correlation with grain yield under water stressed  environments. A split plot design in lattice (16 x 16) arrangement was used with two replications, where three irrigation treatments (WW, WSF and WSG) were allotted to main plots and genotypes (254 top crosses) to sub-plots. A separate analysis of variance of RCBD was also performed under each irrigation treatment for estimating the genetic parameters. The PCV and GCV estimates were high for plant height (PH), ear height (EH) and grain yield/plant (GYPP), low for other studied traits, except for barren stalks (BS) which was of medium magnitude. The highest h2 estimate (>90%) was exhibited by days to anthesis (DTA), days to silking (DTS), PH, EH and leaf rolling (LR) under all environments and anthesis silking interval (ASI) under WW, while the lowest h2 estimate (< 46%) was shown by BS and ears plant-1 (EPP) traits. For DTA, DTS, BS, EPP, GYPP and grain yield/ha (GYPH) traits, heritability was increased in stressful environments (WSF or WSG), while for ASI and LR, the opposite was true. The highest GA (>30%) was shown by PH followed by EH and GYPP, while the lowest GA (<1%) was shown by EPP. The best selection environment for GYPP and GYPH was the stressed one (WSF or WSG). GYPH or GYPP of top crosses showed significant and negative genetic correlations with DTA, DTS, ASI, BS and LR and significant positive correlations with EPP and PH in all environments. [5]

Reference

[1] Doebley, J., 2004. The genetics of maize evolution. Annu. Rev. Genet., 38, pp.37-59.

[2] Lawrence, C.J., Dong, Q., Polacco, M.L., Seigfried, T.E. and Brendel, V., 2004. MaizeGDB, the community database for maize genetics and genomics. Nucleic acids research, 32(suppl_1), pp.D393-D397.

[3] Schadt, E.E., Monks, S.A., Drake, T.A., Lusis, A.J., Che, N., Colinayo, V., Ruff, T.G., Milligan, S.B., Lamb, J.R., Cavet, G. and Linsley, P.S., 2003. Genetics of gene expression surveyed in maize, mouse and man. Nature, 422(6929), pp.297-302.

[4] Daniel, I.O., Adetumbi, J.A., Oyelakin, O.O., Olakojo, S.A., Ajala, M.O. and Onagbesan, S.O., 2012. Application of SSR markers for genetic purity analysis of parental inbred lines and some commercial hybrid maize (Zea mays L.). Journal of Experimental Agriculture International, pp.597-606.

[5] Al-Naggar, A.M.M., Abdalla, A.M.A., Gohar, A.M.A. and Hafez, E.H.M., 2016. Heritability, genetic advance and correlations in 254 maize doubled haploid lines× tester crosses under drought conditions. Archives of Current Research International, pp.1-15.

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