Christian Pichot1, Mohamed El Maâtaoui2, Benjamin Liens1, Juana Laura Rivera Nava1.
Unusual reproductive features in Mediterranean cypresses, and production of all paternal seedlings from a surrogate mother.
in The Paradox of Asexuality: An Evaluation.
- An international joint European Science Foundation
- Linnean Society meeting entitled PARTNER-4,
Linnean Society, London, 22-24th September, 2005.
1. INRA, Unité de Recherches Forestières Méditerranéennes, Avenue Vivaldi, Avignon, France;
2. UMR INRA/UAPV Ecologie des Invertébrés, Faculté des Sciences, 33 Rue L. Pasteur, 84000 Avignon, France.
The analysis of genetic diversity of some Mediterranean conifers led us to discover several ‘abnormalities’ in the reproductive biology of cypresses. In gymnosperms, the diploid embryo is usually surrounded by a haploid megagametophyte which comes from only one or the 4 spores produced by meiosis. In Cupressus sempervirens L. the 4 spores persist and contribute to the megagametophyte development which consequently contains the whole maternal genome. Moreover, cell and nuclear fusions produce a highly polyploid tissue. This feature also occurs in several other Cupressaceae species.
Even more surprising abnormalities were observed in Cupressus dupreziana A. Camus. This species is a highly endangered Mediterranean conifer which survives in the Tassili N’Ajjer desert, south-eastern Algeria, where annual rainfall averages 30 mm. Only 231 trees, most of them very old, were recently scored. C. dupreziana megagametophyte is a diploid based tissue which led us to hypothesize an apomictic origin of the embryos. The lack of maternal alleles in embryo zymograms suggested a strictly paternal origin of embryo nuclear DNA. C. dupreziana was known to produce the largest pollen grains among cypresses. We demonstrated that an abnormal microsprogenesis leads to diploid pollen. C. dupreziana pollen’s ability to produce a (diploid) embryo was demonstrated by pollination of C. sempervirens seed trees with C. dupreziana pollen. All the seedlings were morphologically and genetically similar to the pollen tree. Reverse pollination, revealed the C. dupreziana surrogate ability and leads to the production of all paternal (C. sempervirens) seedlings, most of them haploid (2/3), the other third diploid.
All the analyzed trees exhibited the same abnormalities, but the findings deserve to be confirmed for all the surviving trees. We hypothesize that this ‘paternal apomixis’ occurs in all these trees. Such a strategy may have been selected in response to population size reduction. It avoids consanguinity and inbreeding observed in allogamous species. In this context, we expect a very low genetic diversity among trees, but also a high (or classical) level of heterozygosity. RAPD and AFPL analyses confirmed the low genetic diversity. More recently, nuclear microsatellites markers confirmed the heterozygosity of the trees. C. dupreziana is obviously a unique model system (at least in conifers) for studying the demo-genetic and sexual processes that occur in species prone to extinction.