Search for the presence of active transposons in Indian maize germplasm and assorted genetic testers
-- B.M. Prasanna

The pervasiveness of genetically active mobile elements, such as Uq and Mrh, in maize breeding material as well as lines native to Mexico and South America, and the significance of such activity in maize breeding material, has been emphasized earlier by maize workers. Keeping this in view, a program is being carried out to screen the available Indian germplasm for the presence of active mobile elements. The material includes populations being used for the development of inbred lines, CM lines (Coordinated Maize inbreds, developed by the All India Coordinated Maize Improvement Project, redesignated as Directorate of Maize Research), assorted genetic stocks and a few entries from the North Eastern Himalayan (NEH) maize collection.

The crossing scheme to uncover the active regulatory elements in populations/lines/inbreds, as per standard procedures, involved crosses with the Mobile Element Receptor Lines (MERL), such as c-ruq, a-ruq, a-mrh, a-m(r), bz-rcy and o2-m(r). The colored F1s from crosses of lines x MERL were handled by selfing or backcrossing to the same MERL or a recessive line to uncover the specific recessive used with MERL. The exposure of any MERL to an active regulatory element triggers a spotting pattern in the resultant progeny. Salient results from this study are presented in Table 1.

The number of plants analyzed for each of the entries in this assay exceeded fifteen, except in cases of 1681 (for c-ruq) and CM 105 (for a-ruq) to obtain a 0.95 probability of detecting an element if it is present at a frequency of at least 0.1 (Sedcole, Crop. Sci. 17:667-668, 1977).

Table 1. Test for presence of active mobile elements in Indian maize germplasm and assorted genetic testers: (+) /(-) indicate presence/absence of active element in the material tested.
Corresponding mobile element
Maize line/population MERL Uq Mrh En Cy Bg
Germplasm/Breeding Material
M-18, NEH (Meghalaya strain)1 c-ruq (-)
S-38, NEH (Sikkim strain)1 a-ruq (-)
S-54, NEH germplasm (Sikkim strain)1 bz-rcy (-)
a-mrh (-)
1672 Population2 a-ruq (-)
a-mrh (-)
a-m(r) (-)
o2-m(r) (-)
1681 Population2 c-ruq (+)
bz-rcy (-)
a-m(r) (-)
o2-m(r) (-)
CM 111, Inbred2 c-ruq (+)
CM 105, Inbred2 a-ruq (-)
Genetic Stocks
g r-g, genetic tester1 a-ruq (+)
a-mrh (-)
cl sh1 wx A1 A2 C2 R, genetic tester3 c-ruq (+)
Chr. 3L genetic tester3 bz-rcy (-)
Chr. 7S genetic tester3 c-ruq (-)
1Source: Maize Genetics Section, IARI
2Source: Directorate of Maize Research
3Source: Maize Genetics Coop., USA

The investigation affirms the pervasiveness of the Ubiquitous element in maize germplasm in diverse geographic regions. Although the manner of conscious selection for homozygosity and uniformity should quickly eliminate active mobile elements from the inbred lines, it is surprising to find the presence of an active Uq in one of the two inbreds tested, CM 111, a commonly used inbred line in the Indian maize breeding programs.

The present study utilized only five of the many genetically characterized transposable element systems in maize and also a limited number of entries from the vast NEH germplasm collection. The manner of testing precluded the possibility of ascertaining whether individual plants that showed presence/absence of mutability for a specific element system such as Uq harbor any other known or unknown mobile elements. Also, further tests are needed to find out whether the mutability pattern uncovered in material such as CM 111 (for Uq) is due to the activation of the resident Uq or due to a change at the receptor element. The study is being continued, involving more Indian maize breeding material and NEH lines, to investigate the presence of active mobile elements and their influence in breeding programs. 

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