Botany and Improvement 39
Drew et al. (1998) developed the protocols involving biotechnological skills for
hybridisation of papaya with related Carica species that are resistant to PRSV-P
(C. cauliflora, C. quercifolia and C. pubescens). A highly efficient protocol was
used for rescue and germination of C. papaya × C. cauliflora immature embryos.
Embryos were made to germinate from embryogenic cultures on hormone-free agar
solidified medium and multiple hybrid plants were produced. The C. papaya × C.
cauliflora hybrids lacked vigour and were generally, infertile. Subsequently, the
protocol was adopted to produce hybrids between C. papaya and other PRSV-P
resistant species, C. quercifolia and C. pubescens. Hybrid plants grew vigorously
in the field and a few C. papaya × C. quercifolia plants produced some viable pol-
lens. Inter-specific hybrid plants between C. papaya, C. goudotiana and C. parvi-
flora were also produced. C. parviflora was hybridised with C. pubescens and C.
gaudotiana. Plants of all these crosses could be grown in glasshouse or in field.
It is envisaged that these procedures may also allow access to other characteris-
tics of wild species, such as Phytophthora palmivora resistance (C. gaudotiana),
high sugar content (C. quercifolia) and cold tolerance (C. pubescence) (Drew et al.
1998). Magdalita et al. (1998) have also described an efficient protocol for inter-
specific hybridisation between C. papaya L. × C. cauliflora Jacq. that involved the
use of highly viable pollen of C. cauliflora produced during summer, autumn and
spring; the use of an isolation time ranging from 90–120 days post pollination of
hybrid embryos and the use of most compatible C. papaya cv., 2001 for crossing
with C. cauliflora. The hybrids developed this way are reported to be resistant to
PRSV-P (Ray 2002).
2.7.5 Polyploidy
A diploid commercial dwarf cultivar, Wonder Blight and 40 anther-derived papaya
strains were raised in the same greenhouse. Morphological data based on sizes of
stems, leaves, flowers, fruits, parthenocarpic ability and fruit yield were also col-
lected. The anther-derived papaya strains turned out to be all female, but were vari-
able in ploidy and morphology. The anther-derived plants were of different ploidy,
namely haploids, diploids, triploids and tetraploids. Morphologically, even plants of
the same ploidy were variable in height, parthenocarpic ability, fruit size, shape and
yield. To conclude, the female papaya plants derived from the male gametophyte
originated from the microspores. The haploids and diploids are very useful homo-
zygous breeding lines, while the high-yielding triploids and tetraploids have a lot
of potential for exploitation in commercial production of seedless fruits (Rimberia
et al. 2009).
Polyploidy has received considerable attention in papaya breeding programme.
Hofmeyr (1942, 1945) was able to induce polyploidy in papaya. He found that the
quality of tetraploid fruit was better than the diploid and the fruit was also compact
with smaller seed cavity, but tetraploids were observed to be less fertile than dip-
loids. Singh (1955) reported complete sterility in both female and male tetraploids
and expressed doubt about their commercial utilisation. Later, Zerpa (1957) reported
colchicines-induced tetraploid hermaphrodite plants which were used as male par-
ent in a cross with a female diploid and the tetraploid produced a few seeds without