Evaluation of Ceiba Pentandra for stem dieback disease resistance and characterization by molecular markers

Ceibapentandra (L.) Gartn is an important multi-purpose tree species in Ghana and demand for it is rising daily. However, the occurrence of stem dieback disease at the nursery and plantations, coupled with the absence of adequate information on resistance levels and the genetic diversity within and among populations, has limited the success of efforts geared at devising efficient strategies for its sustainable management. This study assessed resistance levels through progeny evaluation at both the nursery and in the field. Narrow sense heritability and genetic gain in stem height were also estimated. The diversity among accessions was characterized using five morphological traits. The genetic diversity and gene flow within and among five populations of the species were also studied using Random Amplified Polymorphic DNA (RAPD) and Inter-Simple Sequence Repeat (ISSR) markers. The study also looked at markers that are linked to stem dieback resistance. Screening at the nursery from September to November (SNS) showed survival level ranged from 0.20% to 79.20 %. March to May screening (MMS) showed that all the 59 accessions screened had varied levels of survival ranging from 24.4 % to 100 %. Mortality rate was generally higher in the SNS than the MMS. Field study showed that 5 (13.51 %), 9 (24.33%) and 23 (62.16 %) accessions, out of 37 screened, had survival levels between 70.00 and 100 %, 50.00 and 69.90 % and below 50.00 %, respectively. There were significant differences in mean mortality rates among accessions in the nursery and field studies and also in mean stem height and mean diameter growth (P < 0.001). Narrow sense heritability was 0.56 and genetic gain in height ranged from -29.58 to 20.89 cm/yr. Mean population diversity index, using the Shannon Information Index (I), was 0.425 ± 0.024, 0.306 ± 0.027 and 0.371 ± 0.018 for the RAPD, ISSR and the combined RAPD and ISSR, respectively. Gene flow estimates showed low to moderate differences among populations with Gst values at 0.0751, 0.0736 and 0.0799 for the RAPD, ISSR and combined RAPD and ISSR, respectively. Ninety-eight percent of the population differentiation was attributed to within population variation with RAPD polymorphism. ISSR and the combined RAPD and ISSR both showed that a high proportion of genetic diversity resided within populations. Chi-square and G-square tests showed differences among population. Percentage polymorphic loci, Ewens-Watterson Neutrality Test, Nei diversity and Shannon diversity indices, showed Dry Semi-deciduous Forest Inner Zone as the most diverse population. Principal component analysis, defined by axes 1 and 2, accounted for 67.15% of the total variation among accessions. None of the ten markers used was able to differentiate between resistant and susceptible accessions. Nine accessions were identified for conservation as seed trees. Dry Semi-deciduous Forest Inner Zone (DSDFZ-Inner) < Dry Semi-deciduous Forest Outer Zone (DSDFZ-Outlier) < Guinea Savanna Zone (GSZ) < Moist Semi-deciduous Forest Zone (MSDFZ) < Moist/Wet Evergreen Forest Zone (M/WEFZ), in order of decreasing importance, should be preferred in C. pentandra seed collection expedition. Decision making for long-term conservation of C. pentandra should be made on the basis of both morphological and molecular considerations. Sampling for seed trees should be concentrated on selecting resistant and unrelated accessions within populations rather than collecting bits from the entire range of the species. Known resistant and susceptible lines should be used in further assessment for dieback resistant markers. A large number of dominant markers and the use of co-dominant markers should also be investigated.

Source: KNUST