THESIS

2) Cross - compatibility of S. torvum and other three species (S. americanum,
S. villosum and S. nigrum)
These cross combinations among S. torvum, S. americanum, S. villosum
and S. nigrum were not successful when used S. torvum as female parent but when S.
torvum was used as male parent were successful crossable. These indicated that the
recipocal crosses could be overcome the incompatibility crossing barrier (Jansky,
2006). This was also showed the incompatibility in S. torvum. Novy and Hanneman
(1991) noted that pollen tubes of S. brevidens and S. fernandezianum were
consistently inhibited in the upper portion of the styles of self-incompatible of haploid
Tuberosum Group.
When S. torvum was used as male parent, the highest percentage of fruit
setting (78.0) was obtained in the cross between S. villosum and S. torvum (TS02600
x S00429) but no hybrid seeds were obtained (Table 17). The cross between S.
americanum (S00859, collected in Tanzania) and S. torvum (S00429, collected in
Thailand) gave the fertile hybrid seed. Furthurmore, the abortive hybrid seeds were
obtained from the crossing between S00861 (collected in Japan) x S00429 (collected
in Thailand) (Table 17). Similarly, the crosses between S. villosum (S00854, collected
in Tanzania) x S. torvum (S00429) yielded the fertile hybrid seeds but no seed was
obtained from the crosses between S00860 (collected in Japan) x S00429. This result
indicates that the level of cross compatibility depends on the genotype and/or on
geographical origins (Hawtin et al., 1997).
However, the crosses between S. nigrum and S. torvum gave hybrid seeds
but no hybrid plants were obtained because of the abortive seeds. Similar to the
crosses between S. villosum and S. torvum hybrid seeds were obtained without hybrid
plants. This result due to the chromosome number of S. villosum (2n = 48) and S.
nigrum (2n = 72) were different from S. torvum (2n = 24), while S. americanum (2n =
24) had a same chromosome number with S. torvum (Edmond and Chweya, 1997). In
addition, the variation of hybrid plants were obtained depend on the genotype of
parents (Singh et al., 2002).
81