Describe how transposition can give rise to variegated (speckled) maize kernels
ID: 260724 • Letter: D
Question
Describe how transposition can give rise to variegated (speckled) maize kernels by answering the following questions:
a) Which gene is responsible for the purple kernel color seen in wild-type kernels?
b) How do white kernels occur?
c) How do purple speckled kernels occur?
d) If the transposable element of maize utilized the mechanism used by Tn3 (replicative transposition), would you expect to see speckled kernels at the same high frequency? (*Note* This hypothetical assumes all the regular elements (Ds, Ac, inverted repeats, etc) are present, but the transposition is done via replicative transposition.)
Explanation / Answer
Corn kernels colour variations occur through methods of transposition. There are four genes that play a role in these colours, resulting in yellow, purple or red colours. The pathway involved in this work is the anthocyanin pathway, that give rise to either purple pigments or red pigments. The last colour occurs as a result of no pigments present in the kernel. The transcription activators lead to the synthesis of these colours. The biochemical phenotype of purple colour occurs by the Pr gene with protein involved as the flavonoid 3’ hydroxylase present at the 5L chromosomal location. The Pr1 allele plays the role in purple pigment formation, the cyanidin pigment.
White endosperms appear because the kernels having y1y1y1 white endosperm would be lacking the enzyme phytoene synthase, which is an essential enzyme required in the initial stages of the biosynthetic pathway for synthesis of the phtoene. Precursors being accumulated are basically colourless so that the endosperm that are being formed will be white in colour. Though, the embryo being formed are still yello in colour.
Purple pigment is formed from the major lament cyanidin 3O glycoside. There is presence of an Ac/Ds system that cause the genes to jump. The Bz is the dominant allele that would be producing the purple colour and th bz allele produces the purple brown to dark brown colour. Ds moves into the normal Bz allele leading to a mutation in the colour. Even after coding with the dominant colourless allele, the phenotype was of the recessive type, which occurred due to breakage. Breakage occurred due to the activator element (Ac) element, leading to this deviant phenotype.
Replicative transposition would increase the rate of transposition of the elements.