Cacao swollen shoot virus detection and DNA barcoding of its vectors and putative vectors in Theobroma cacao L. by using polymerase chain reaction.

Cacao swollen shoot virus (CSSV) is an endemic pathogen causing significant economic losses to cacao (Theobroma cacao L.) production in West Africa. There is limited updated report on the occurrence, spread, genetic diversity and species of CSSV and its mealybug vectors, especially in Nigeria. Nigeria is presently lagging behind in the search for resistance to CSSV and its vectors in T. cacao L. The present study aimed to map and screen for the presence of CSSV and its natural vectors - female mealybugs (Pseudococcidae: Hemiptera) in cacao plantations in Nigeria. Symptomatic and asymptomatic cacao leaves and whole female mealybug samples were collected from major cacao-growing areas in Nigeria - Abia, Akwa Ibom, Cross River, Edo, Ondo and Oyo States. A total of 2568 cacao leaves from 1052 cacao trees were screened with polymerase chain reaction (PCR) using an open reading frame 1 (ORF 1) CSSV-specific primer pair. PCR screening of the mealybug species was performed using the cytochrome c oxidase subunit I (COI) gene. A combination of scanning electron microscopy (SEM) and histology for morphological identification and DNA barcoding enabled to characterise the female mealybug species. The results revealed that CSSV and its mealybug vectors are present in the major cacaogrowing areas in Nigeria. Although CSSV and its vectors have been previously reported in Cross River, Ondo and Oyo States, our results present the first documented evidence of CSSV emergence and its mealybug vectors in Abia, Akwa Ibom and Edo States. We also present the first report of Pseudococcus jackbeardsleyi (Gimpel and Miller) mealybug species on cacao in Nigeria. In conclusion, it is pertinent to re-establish coordinated routine survey and monitoring of CSSV and its mealybug vector presence in T. cacao L. in Nigeria.

Progressive erosion of genetic and epigenetic variation in callus-derived cocoa (Theobroma cacao) plants.

*Relatively little is known about the timing of genetic and epigenetic forms of somaclonal variation arising from callus growth. We surveyed for both types of change in cocoa (Theobroma cacao) plants regenerated from calli of various ages, and also between tissues from the source trees. *For genetic change, we used 15 single sequence repeat (SSR) markers from four source trees and from 233 regenerated plants. For epigenetic change, we used 386 methylation-sensitive amplified polymorphism (MSAP) markers on leaf and explant (staminode) DNA from two source trees and on leaf DNA from 114 regenerants. *Genetic variation within source trees was limited to one slippage mutation in one leaf. Regenerants were far more variable, with 35% exhibiting at least one mutation. Genetic variation initially accumulated with culture age but subsequently declined. MSAP (epigenetic) profiles diverged between leaf and staminode samples from source trees. Multivariate analysis revealed that leaves from regenerants occupied intermediate eigenspace between leaves and staminodes of source plants but became progressively more similar to source tree leaves with culture age. *Statistical analysis confirmed this rather counterintuitive finding that leaves of 'late regenerants' exhibited significantly less genetic and epigenetic divergence from source leaves than those exposed to short periods of callus growth.

Headspace volatile markers for sensitivity of cocoa (Theobroma cacao L.) somatic embryos to cryopreservation.

The mechanisms that reduce the viability of plant somatic embryos following cryopreservation are not known. The objective of the present study was to evaluate the sensitivity of cocoa (Theobroma cacao L.) somatic embryos at different stages of an encapsulation-dehydration protocol using stress-related volatile hydrocarbons as markers of injury and recovery. The plant stress hormone ethylene and volatile hydrocarbons derived from hydroxyl radicals (methane) and lipid peroxidation (ethane) were determined using gas chromatography headspace analysis. Ethylene and methane were the only volatiles detected, with both being produced after each step of the cryogenic protocol. Ethylene production was significantly reduced following exposure to liquid nitrogen, but then increased in parallel with embryo recovery. In contrast, the production of methane was cyclic during recovery, with the first cycle occurring earlier for embryos recovered from liquid nitrogen and desiccation than those recovered from earlier steps in the protocol. These results suggest that loss of somatic embryo viability during cryopreservation may be related to the oxidative status of the tissue, and its capacity to produce ethylene. This study has demonstrated that headspace volatile analysis provides a robust non-destructive analytical approach for assessing the survival and recovery of plant somatic embryos following cryopreservation.

Detection and quantification of in vitro-culture induced chimerism using simple sequence repeat (SSR) analysis in Theobroma cacao (L.).

Mutation rates are often elevated in plants regenerated from in vitro culture, giving rise to so-called 'somaclonal variation'. Detailed characterisation of mutation profiles that arise during culture should improve our understanding of processes influencing mutation and allow the selection of protocols yielding the fewest/least severe changes. Somatic mutations will usually produce genetic chimeras where unchanged alleles are retained by some cells. Such chimeras are difficult to detect but likely to form a significant proportion of any regenerant population. We present a simple protocol that enables the provisional diagnosis of both homogenous and chimeric mutants among large regenerant populations, together with a semi-quantitative means of estimating the proportion of mutant cells. The assay exploits consistent differential amplification of alternate simple sequence repeat alleles at heterozygous loci. Calibration of the relative amplification of alleles from two genotypes-and the synthetic chimeras created from them-revealed a strong linear relationship between 'peak heights' representing alternate alleles following capillary electrophoresis. The assay predicts chimeric composition to a reasonable level of confidence (+/-5%) so long as the infrequent allele exceeds 15% of the template. The system was applied to 233 regenerants of cocoa somatic embryogenesis and identified 72 (31%) putative chimeric mutants for slippage mutation or allele loss across two loci.

Suitability of cryopreservation for the long-term storage of rare and endangered plant species: a case history for Cosmos atrosanguineus.

The suitability of cryopreservation for the secure, long-term storage of the rare and endangered species Cosmos atrosanguineus was investigated. Using encapsulation/dehydration of shoot tips in alginate strips, survival rates of up to 100 % and shoot regeneration of up to 35 % were achieved. Light and electron microscopy studies indicated that cellular damage to some regions of the shoot tip during the freeze/thaw procedure was high, although cell survival in and around the meristematic region allowed shoot tip regeneration. The genetic fingerprinting technique, amplified fragment length polymorphisms (AFLPs), showed that no detectable genetic variation was present between material of C. atrosanguineus at the time of initiation into tissue culture and that which had been cryopreserved, stored in liquid nitrogen for 12 months and regenerated. Weaned plantlets that were grown under glasshouse conditions exhibited no morphological variation from non-frozen controls.