Botany and Improvement 41
Genetic variability with respect to disease resistance in Carica papaya L. is low.
In vitro culture techniques have greatly helped in producing mutants and variants
that are being used in breeding programmes in different countries. Cybrids with
good attributes can be produced by protoplast fusion with other species of Carica
(Manshardt 1992). Molecular markers, DNA finger printing and genome mapping are
being used to identify the species, their mixtures and their contribution to the prog-
enies in the hybridisation programmes (Manshardt 1992; Sharon et al. 1992; Sondur
et al. 1996; Magdalita et al. 1998) and also to variation in the sex forms within the
species (Nandi and Mazumdar 1990; Stiles et al. 1993; Manshardt and Drew 1998).
A RAPD map of the papaya genome has been generated and used to identify mark-
ers for sex determination, flowering height and fruit carpelloidy (Sondur et al. 1996).
Somsri et al. (1998) have compared RAPD (Randomly Amplified Polymorphic
DNA) and DNA amplification finger printing (DAF) to develop molecular mark-
ers for sex prediction in papaya. They observed that DAF produced at least five
times more bands than equivalent RAPD reactions permitting more efficient predic-
tion. Preliminary analysis for linkage associations indicated that these markers were
closely linked to the sex-determining alleles. Conversion of some DAF markers into
more convenient SCAR markers proved difficult since DAF bands were difficult to
clone (Somsri et al. 1998).
Phylogenic analysis based on isozyme and DNA polymorphisms indicate that
papaya is a distant relative of other members in the genus. Jobin-Decor et al. (1997)
have reported that C. papaya is about 70% dissimilar to other Carica spp. Wild spe-
cies like, C. pubesceus and C. stipulata are much closer to each other with similarity
of 87% by isozyme analysis and 82% by RAPD analysis (Jobin-Decor et al. 1997). In
order to produce inter-specific crosses where the parents are taxonomically distant,
barriers that prevent successful hybrid production have been overcome by in vitro
embryo culture. Yung (1986) and Fitch and Manshardt (1990) succeeded in regen-
erating papaya plants from immature zygotic embryos through somatic embryogen-
esis. Magdalita et al. (1996) improved embryo-rescue protocol for younger (90 days
old) embryos of C. papaya L. (clone 2001), and subsequently utilised the technique
for efficient production of inter-specific hybrids of C. papaya × C. cauliflora from 90
to 120-days-old embryos. The relative ease with which Carica inter-specific hybrids
can be produced by embryo or ovule rescue (Drew et al. 1998) has virtually directed
research activity away from protoplast fusion. However, production of dihaploid
inbred lines through anther culture is still under-researched area that merits more
attention.
Unfortunately, in most countries, papaya suffers from PRSV, limiting its pro-
ductivity commercially as well as in the backyard (Gonsalves et al. 2007). The
developers of the first transgenic papaya envisaged the GE variety as a promising
pro-poor product of biotechnology and were eager to collaborate with researchers
from around the developing world. Suitable GE, virus-resistant varieties have now
been developed for Brazil, Jamaica, Venezuela, Thailand, China and the Philippines
among other countries. Yet, in no place outside Hawaii have growers or consumers
reaped the benefits of these plants. Gonsalves and her colleagues (2007) highlighted
that this technology is particularly suitable for low-income farmers. With regard to
consumer demand, the nutritional value of papaya, while important to Hawaiian