Compatibility and Genetic Studies of Some Carica Species and Their Hybrids

Abstract

Experiments were conducted to study floral biology, cross compatibility, heterosis, cytogenetics and inheritance of several morphological characteristics among nine Carica species: Carica cauliflora, C. goudotiana, C. monoica, C. parviflora, C. pennata, C. pubescens, C. papaya, £. horovitziana and Index number 203. All species studied except C. parviflora showed maximum anthesis between 4 and 10 p.m. C. parviflora showed maximum anthesis between 8 to 10 a.m. The hybrid, C. parviflora (8 - 10 a.m. anthesis time) x goudotiana (4 - 6 p.m.) showed maximum anthesis between 10 to 12 noon. Pollen was successfully germinated at six pH levels ranging from 4.5 - 9.5. A gradual rise in percentage germination of pollen from pH 4.5 - pH 7.5 followed by a sharp drop at pH 8.5 and pH 9.5 was observed in all the species. Results also showed significant differences between species. Pollen of C. goudotiana and C. papaya showed lower geraination percentages than pollen of other species. All species were self-compatible. Cross compatibility studies showed crosses that were fertile, crosses which set fruit but produced empty seeds, crosses that set parthenocarpic fruits and crosses that failed to set fruit. C. papaya was cross incompatible with all species used in this study. A new hybrid C. parvif lora x C. goudotiana was produced. A description of ten hybrids developed in this work was presented. All hybrids were fertile. Heterosis for vegetative growth and productivity was conclusively shown in two crosses. Comparisons were made between the means of hybrids and those of their high parents. Stem color and petiole color were each shown to be under the control of a single gene pair, red being dominant to green in both cases. Variations in the intensity of red color in stems and petioles were attributed to the possible influence of modifier genes. Monogenic control was established for flower color with purple-blush and white, both dominant to pale yellow flower color. Red ripe skin color of the fruit was dominant to yellow. A cross between plants of red fruit color with those of orange fruit color produced hybrids with pink colored fruits, indicating lack of dominance between red and orange. Fruit ridges were dominant to wide grooves which in turn were dominant to narrow grooves. Mode of inheritance was monogenic in both cases. Spiny and non-spiny seed coats were simply inherited with dominance lacking. A single-gene difference between succulent fruit pulp and dry pulp with recessiveness for the latter was shown. A dominant monogenic control of profuse branching over sparse branching was observed. Crosses between dioecious and monoecious species using pollen from the latter produced only monoecious F₁ plants. The reciprocal cross produced F₁ plants in an approximate ratio of 1 male: 1 monoecious. Differential suppression of androecium observed in crosses between C. monoica and C. goudotiana requires further investigation. Cytological observations confirmed nine pairs of chromosomes for all species and hybrids investigated. Occasional partial pairing and subsequent precocious anaphasic separation involving one pair of homologues were observed in all species and hybrids studied. Heteromorphism between these two homologues was detected in three species and three hybrids. These findings seem to support the hypothesis that sex chromosomes do exist in these species and hybrids. Chromosome lagging sometimes observed at anaphase I in two of the hybrids was a new phenomenon in the genus. This feature could result in the evolution of aneuploids in the genus. High fertility of hybrids was attributed to regularity of bivalent formation. This is suggestive of close genetic affinity between the parent species. Speculations into the potentials of the wild Carica species were discussed.