Cylicomorpha

Abstract: The chloroplast and mitochondrial DNA diversity of 61 genotypes belonging to 18 Vasconcellea species, the so-called highland papayas, was studied by PCR-RFLP analysis of two non-coding cpDNA regions (trnM-rbcL and trnK1-trnK2) and one non-coding mtDNA region (nad4/1-nad4/2). This sample set was supplemented with six genotypes belonging to three other Caricaceae genera: the monotypic genus Carica, including only the cultivated papaya, and the genera Jacaratia and Cylicomorpha. Moringa ovalifolia was added as an outgroup species. The PCR-amplified cpDNA regions were digested with 18 restriction endonucleases, the mtDNA region with 11. A total of 22 point mutations and four insertion/deletions were scored in the sample. A higher level of interspecific variation was detected in the two cpDNA regions in comparison to the analysis of the mtDNA. Wagner parsimony and Neighbor-Joining analysis resulted in dendrograms with similar topologies. PCR-RFLP analysis supported the monophyly of Caricaceae, but among the 26 mutations scored, an insufficient number of markers discriminated between the different Caricaceae genera included in this study. Hence the inference of the intergeneric relationships within Caricaceae was impossible. However, some conclusions can be noted at a lower taxonomic level. The Caricaceae species were divided into two lineages. One group included only Vasconcellea spp., whereas the second included the remaining Vasconcellea spp., together with the papaya genotypes and those from the other Caricaceae genera. This may indicate a higher level of inter-fertility for the Vasconcellea species from the latter clade in interspecific crossings with papaya. The putative progenitors of the natural sterile hybrid V. × heilbornii, i.e. V. stipulata and V. cundinamarcensis, were only distantly related to V. × heilbornii. This indicates that probably none of these species was involved as the maternal progenitor in the origin of V. × heilbornii. Surprisingly, V. × heilbornii had organellar genome patterns identical with V. weberbaueri, suggesting a possible involvement of this species in the origin of V. × heilbornii. On the basis of discrepancy between morphological traits and the cpDNA profiles of some pairs of Vasconcellea species, we believe that besides V. × heilbornii, some other species have originated through interspecific hybridization. A reticulate evolution for Vasconcellea has therefore been suggested. Finally, intraspecific cpDNA variation was detected in V. microcarpa, thus providing molecular evidence for the high diversity previously indicated by morphological observations.

Abstract: PREMISE OF THE STUDY: Caricaceae include six genera and 34 species, among them papaya, a model species in plant sex chromosome research. The family was held to have a conserved karyotype with 2n = 18 chromosomes, an assumption based on few counts. We examined the karyotypes and genome size of species from all genera to test for possible cytogenetic variation. METHODS: We used fluorescent in situ hybridization using standard telomere, 5S, and 45S rDNA probes. New and published data were combined with a phylogeny, molecular clock dating, and C values (available for similar to 50% of the species) to reconstruct genome evolution. KEY RESULTS: The African genus Cylicomorpha, which is sister to the remaining Caricaceae (all neotropical), has 2n = 18, as do the species in two other genera. A Mexican clade of five species that includes papaya, however, has 2n = 18 (papaya), 2n = 16 (Horovitzia cnidoscoloides), and 2 n = 14 (Jarilla caudata and J. heterophylla;third Jarilla not counted), with the phylogeny indicating that the dysploidy events occurred similar to 16.6 and similar to 5.5 million years ago and that Jarilla underwent genome size doubling (similar to 450 to 830-920 Mbp/haploid genome). Pericentromeric interstitial telomere repeats occur in both Jarilla adjacent to 5S rDNA sites, and the variability of 5S rDNA sites across all genera is high. CONCLUSIONS: On the basis of outgroup comparison, 2n = 18 is the ancestral number, and repeated chromosomal fusions with simultaneous genome size increase as a result of repetitive elements accumulating near centromeres characterize the papaya clade. These results have implications for ongoing genome assemblies in Caricaceae.

Abstract: Papaya belongs to the Caricaceae, a family of dicotyledonous, tree-like plants that is made up of four genera, three of which, Carica, Jacaratia, and Jarilla are native to tropical America, while Cylicomorpha is native to Africa (Badillo 1967, 1971). The genus Carica consists of about 21 species, but only C. papaya is of economic importance (Litz 1986a). The ripe fruits of other species are edible, for example, C. pubescens Lenne et Koch (“candamarcensis”), C. monoica Desf., C. erythrocarpa Heilborn., C. × heilbornii nm. pentagona (babaco), C. quercifolia St. Hil (Heiron.), and C. goudotiana Solms-Lauback, but they are more often consumed in preserves (Purseglove 1968; Jordan et al. 1982; Litz 1984).

Abstract: Papaya (Carica papaya L.) is cultivated in all continents producing 12.41 million tonnes from 434,785 ha, but major share of its production comes from Asia, Central America and Africa. Yield of papaya varies from place to place mainly due to the widespread incidence of viral diseases. The genus Carica is more vulnerable to diseases due to narrow gene pool. Among various viral diseases affecting papaya cultivation, Papaya ringspot virus type Papaya (PRSV-P), is the most devastating one in all major papaya-growing areas. The use of PRSV-P resistant transgenic papaya cultivars has been limited to certain geographical regions. Other approaches of managing PRSV-P have only limited success. Therefore, the approach of introgression of PRSV-P resistance in papaya from PRSV-P resistant wild relatives (Vasconcellea species) by conventional breeding has become the only viable option. All PRSV-P resistant Vasconcellea species attempted to be used as a source of resistant gene(s) had certain limitations. V. cauliflora rarely produced hybrids, and most of them were infertile. V. quercifolia produced resistant hybrids which developed mild virus symptoms with age. V. cundinamarcensis was reported to be consistently resistant against PRSV-P infection under many geographical conditions. But it was not possible to transfer PRSV-P immunity to C. papaya because F1 hybrids were infertile females. The approach of bridge crossing is the latest strategy in developing PRSV-P resistance in C. papaya. V. parviflora which is cross-compatible with both V. cundinamarcensis and C. papaya was used as the bridge species. Stable homozygous hybrids of V. cundinamarcensis x V. parviflora were crossed with C. papaya to obtain PRSV-P resistant hybrid with marketable fruiting qualities. In addition to the genus Vasconcellea, other closely related genera of papaya (Cylicomorpha, Horovitzia, Jacarantia and Jarilla) should also be characterized and screened for variation in the PRSV-P resistance and their cross-compatibility with C. papaya to use them as wild sources in resistant breeding programme.