Botany and Improvement 35
often used for the recovery of desirable traits in such cultivars. Ram (1993) described
the use of backcrossing in the improvement of inbred lines for use in developing
papaya hybrids. For this, suitable male plants are selected from the same progeny
which have resemblance to female plants in vegetative characters, such as stem and
leaf colour, stem thickness and height at flowering. The typical female plant having
maximum resemblance to a cultivar should be crossed with such three male plants
identified earlier for the purpose. After confirmation, the best male plant should be
retained for maximum possible generations for backcrossing. Due to dioecism in
papaya, it may take 18 generations (36 years) to become almost completely homo-
zygous (Hofmeyr 1953). Backcrossing with the retained male plant automatically
reduces this period as it is difficult to retain the plant for such a long period in the
field. Progenies rose from BC 1 or BC 2 inbred lines are screened and desirable female
plants are selected for further sib-mating. The process is to be continued for 7–8
generations to achieve uniformity for a particular character or a group of characters.
In backcrossing of a dioecious strain, seedlings segregate in 1 female: 1 male ratio
(i.e. in equal proportion).
Although backcrossing does have some specific advantages as a breeding tech-
nique, a serious weakness called inbreeding depression is an undesirable fall-out
of its use. The magnitude of inbreeding depression (i.e. decrease in vigour) is not
the same in all lines produced by inbreeding. The point, after several generations
of inbreeding, at which no further decrease of fitness and vigour occurs, is referred
to as the ‘inbreeding minimum’. The crossing of inbred lines which have reached
their inbreeding minimum frequently results in heterosis. The cause of inbreeding
depression is associated with the uncovering of deleterious recessives and lethals in
homozygous genotypes. However, Hamilton’s (1954) study in inbreeding and cross-
breeding of two Solo strains showed that inbreeding has no ill effects on plant vigour.
Breeding for gynodioecious lines involves selfing of a regular and prolific bearing
hermaphrodite or sibmating the female with the hermaphrodite. Suitable hermaph-
rodite plants whose sex does not change with climatic variations are selected. As
done in the case of dioecious lines, here also the desirable female is crossed with at
least three hermaphrodite plants as male parent. Sib-mating in female and selfing in
hermaphrodite may go simultaneously to speed up the breeding programme of the
various types of flowers produced by a hermaphrodite plant; elongata and pentandria
types are selected for selfing. This process is to be continued for 7–8 generations
for satisfactory homozygosity to be achieved. In this method, the sex ratio (female:
hermaphrodite) is found to be 1:1 in sib-mating and 1:2 in selfing of hermaphrodites
(Ram 1996). The major advantage of developing a gynodioecious line is that all
plants bear fruits. The time consumed in attaining complete homozygosity through
backcross breeding is reduced substantially if anther culture is adopted.
2.7.2.2 Recurrent Selection
The basic technique in recurrent selection is the identification of individuals with
superior genotypes, and their subsequent inter-mating to produce a new population.
Some type of progeny test, depending on the selection system, may be necessary
to measure the genetic worth of the parents. Parental genotypes are retained often
by controlled selfing, so that by following the process of selection and progeny