Genetics
Genetic research of this genetic drift started first with describing (phenotyping) the mutant kernel appearance of maize and other mutant changes in the maize. Lateon these description were coupled to mutant genes genotypes. More than 40 mutant alleles are known for the waxy locus, making up the finest collection of mutations found among higher plants.
Some of these waxy mutants are very stable whereas others are very unstable. The genotype of the stable mutants remains unchanged whereas the one of unstable mutants changes because of the insertion of transposable elements (5-8). For a listing of all these mutations, the excellent book of Neuffer, Coe et al. is greatly recommended.
Because the waxy mutation is expressed in an easy identifiable nonlethal phenotype, it has been the subject of major research during the 20th century. Nelson made a fine structure genetic map of most of these mutations
- For waxy maize, a single recessive gene (wx) was located on the short arm of chromosome 9, codes for the waxy endosperm of the kernel (Wx codes for endosperm with normal starch). This was first shown by Collins and Kempton.
- The structure of the wildtype waxy (wx+) locus has been determined through DNA sequencing. The gene has 3718 bp (14 exons and 13 introns).
- Waxy endosperm is the counterpart in maize of the “glutinous” character in rice.
- There is a wide range of species also presenting the waxy mutation, including rice, sorghum, millet, barley and wheat, which were characterised by starch granules staining red with iodine.
- In crosses between heterozygous plants for the waxy character, a small but significant deviation from an expected Mendelian ratio in self-pollination is produced. Bear obtained from 71 segregated ears on the F1 generation 23,77% of waxy kernels and 76,23% of non-waxy kernels. This is evidenced by the two heterozygous types, Wx Wx wx and wx wx Wx.
The waxy gene is epistatic for all known other amylose and amylopectine forming mutants genes like dull (du), sugary-1 (su1 ) and sugary-2 (su2),. wx gene for example increases sugars and water-soluble polysaccharides (WSP) in a su1 background and it causes dramatic increases in sugars and reduction in starch with ae or aedu mutated genes.
- The mutation from Wx to wx is not uncommon in Corn Belt varieties, Bear having found three separate mutations to waxy in three consecutive years in a total population of some 100,000 selfed ears.
- Mangelsdorf found also many mutants on his trial fields.
- Argentine waxy (wx-a) corn, an allele at the waxy locus first reported by Andrés and Bascialli, is known to produce small amount of amylose (< 5%) and gives an intermediate staining reaction with iodine.
- Other mutant alleles at the waxy locus have been reported which possess similar starch properties to those observed with wx.
Read more about this topic: Waxy Corn