PhD

The Brown planthopper (BPH), Nilaparvata lugens (Stal), has become a
serious threat to rice production throughout Asia. Very extensive losses have
occurred in India, Indonesia, and the Phylippines. The most severe outbreak in
India occurred in Kerala during 1973-74 in ‘Kole’ lands of Trichur district
and ‘Kuttanad’ area of Kottayam and Alleppey districts. Although insecticides
provide effective control, this approach is expensive and creates problems of
environmental pollution. Resistant varieties can provide protection and
insurance against this insect pest at no extra cost and with no danger from
chemical residues. Very little work has been done in Kerala to identify sources
of resistance to the local biotype of BPH and on the genetic basis of BPH
resistance. The major objectives of the present investigation were to identify
sources of resistance to BPH and to conduct genetic analysis and understand
the mode of inheritance of BPH resistance.
One hundred and nine rice types were studied for their reaction to BPH
through the bulk seedling test at the seedling stage and tiller test and
honeydew experiment at the tillering stage. Out of them 41 were found to be
resistant, 22 moderately resistant, 13 moderately susceptible and 33 highly
susceptible. In the tiller test, 31 out of the 41 resistant varieties were resistant,
nine moderately resistant and one moderately susceptible. The thirty one types
found resistant under tiller test were subjected to honeydew experiment, out of
which 30 were found to be resistant and one was moderately resistant.
2
The inheritance of resistance was studied in eight types selected from
among the 30 types proved to be resistant in all the three tests. They were
crossed with the susceptible variety TN1 and the F1, F2 and F3 generations
were studied by bulk seedling test, tiller test and honeydew experiment. F1
seedlings were also screened by bulk seedling test, tiller test and honeydew
experiment. The F2 progenies were screened by the bulk seedling test and tiller
test. The F3 seedlings were screened by bulk seedling test only. The F2 and F3
progenies were scored separately as resistant and susceptible types and the
observed segregation ratios were tested against the expected by applying the
test of goodness of fit.
The F1s of all the eight crosses were resistant indicating that resistance
in each of the eight types was governed by dominant gene. The F2 populations
of all the eight crosses segregated in the ratio of 3 resistant : 1 susceptible
indicating that a single dominant gene governed resistance in each of the eight
resistant types. F3 breeding behaviour of the nine F2 resistant plants from each
of the eight crosses confirmed the monogenic control of resistance over
susceptibility. Two dominant genes Bph1 and Bph3 were identified at IRRI
(Bph1 in variety Mudgo and Bph3 in RatuHeenati). In the present study,
Mudgo containing Bph1 and RatuHeenati with Bph3 gene were not resistant.
Hence it is assumed that the dominant resistant gene identified in the present
study is neither Bph1 nor Bph3.
3
Diallele crosses were made between six resistant types selected based on
diverse origin. The F1 and F2 progenies of the 15 combinations were studied to
get information on the allelie relationship between the resistance genes. The F1
progenies of all the crosses were resistant and the F2 progenies were
homogeneous for resistance. This lead to the conclusion that all the six types
have the same dominant gene for resistance. All the six resistant types were
isogenic and hence all of them are expected to carry a dominant gene for BPH
resistance other than Bph1 and Bph3.
The present study has thus made available several types resistant to the
local biotype of BPH and also enabled the location of a new dominant gene
conferring resistance to this biotype. These results and the materials made
available can form the basis for a more effective breeding approach for BPH
resistance in this region.