Marie-France Duval

Molecular diversity in pineapple assessed by RFLP markers

Abstractu2002Pineapple, Ananas comosus (L.) Merr, is the third most important tropical fruit cultivated in all tropical and subtropical countries. Pineapple germplasm includes all seven species of the genus Ananas and the unique species of the related genus Pseudananas. A knowledge of its diversity structure is needed to develop new breeding programs. Restriction fragment length polymorphism (RFLP) was used to study molecular diversity in a set of 301 accessions, most of which were recently collected. This sample was analysed using 18 homologous genomic probes. Dissimilarities were calculated by a Dice index and submitted to Factorial Analysis. The same data were represented as a diversity tree constructed with the score method. Pseudananas sagenarius displayed a high polymorphism and shares 58.7% of its bands with Ananas. Within Ananas, variation appears continuous and was found mostly at the intraspecific level, particularly in the wild species Ananas ananassoides and Ananas parguazensis. As for the cultivated species, Ananas comosus appears relatively homogeneous despite its wide morphological variation and Ananas bracteatus, which is grown as a fence and for fruit, appears still much less variable. By contrast Ananas lucidus, cultivated by the Amerindians for fiber, displays a high polymorphism. This tree displayed a loose assemblage of numerous clusters separated by short distances. Most species were scattered in various clusters, a few of these being monospecific. Some accessions which had not been classified, as they shared morphological traits typical of different species, re-group with one or the other, and sometimes with both species in mixed clusters. No reproductive barrier exists in this germplasm and these data indicate the existence of gene flow, enhancing the role of effective sexual reproduction in a species with largely predominant vegetative mutiplication.

Main results from the EU-funded project "Evaluation and utilization of pineapple genetic resources from the Amazon to breed resistant varieties"

This project aimed at obtaining knowledge and tools for the development of cultivars resistant to the main diseases in South America, by (i) characterizing and evaluating available genetic resources, (ii) studying the genetic structure of the genera Ananas and Pseudananas, (iii) developing and applying resistance screening techniques for important pests and diseases in the region, (iv) studying the heredity of agronomic traits, and (v) testing the potential of partial inbreds in breeding. In addition, a standard database is being established in collaboration with IPGRI, to promote information and germplasm exchange. The project, which started in 1997 and ended in 2001, has produced an inventory of the germplasm held in the partners field collections, allowing exchanges and repatriation of lost germplasm. The list of descriptors has been revised. Multivariate analyses of the morphological data have produced consistent results. Molecular characterization, based on RFLP of genomic DNA and PCR-RFLP of cpDNA has confirmed the proximity of Ananas and Pseudananas, the absence of interspecific barriers in Ananas, the importance of a geographic component of variation, and the existence of a higher genetic diversity in the North of South America. Pineapple taxonomy has been revised. Genetic mapping was based on isozyme, RAPD, AFLP and ISSR markers and carried out on a hybrid progeny between A. bracteatus and A. comosus, following the pseudo-testcross approach. It resulted in a first map of 336 and 154 DNA markers distributed in 46 and 31 linkage groups for these two species. New sources of resistance to fusariosis have been identified, some of which show other interesting traits for the Brazilian producers. According to the first inheritance studies, this resistance seems monogenic and dominant, a result still to be confirmed. Self-progenies obtained from the cultivars Perolera, Primavera, and Roxo de Tefe, showed inbreeding depression, but normal segregation for major leaf traits (spininess, color). Subsequent hybridization of the inbreeds restored vigour. (Resume dauteur)