In the cypress improvement programme for resistance against canker disease, caused by Seiridium cardinale, controlled crosses among cypress species were done by the IPP. The hybrids Cupressus sempervirens (as female) x C. dupreziana (as male) aroused great interest, becuase some morphological traits of hybrids were always identical to those of the paternal tree, C. dupreziana. A team, constituted by Italian and French biologists, geneticists and sylviculturists, examined six 15-year-old families obtained by controlled crosses of 6 different clones of C. sempervirens (as female) and C. dupreziana (as male). C. dupreziana is native to the Tassili N'Ajjer (figure 1) desert of Algeria and is one of the most threatened trees in the world. The following morphological characteristics distintive of the parental species were measured for the hybrids and the parents:
- orientation of terminal twigs : in one plane in C. dupreziana, in all directions in C. sempervirens;
- cone size: always larger in C. sempervirens;
- percentage of filled seed: very low in C. dupreziana;
- endosperm ploidy levels: only even levels:2n, 4n, 6n...in C. dupreziana;
- pollen diameter: 38 ìm in C. dupreziana and 28 ìm in C. sempervirens;
- pollen ploidy level: diploid in C. dupreziana.
For all these characters all the hybrids resulted identical to the male tree, C. dupreziana. Genetic diversity was assessed by using two markers: isozymes in only one hybrid family ( seven polymorphic sistems: Fest, Idh,Lap,6Pgd, Pgi, Pgm and Skdh) and RAPD, four operon primers:OPA-08, OPA-15, OPA-18 and OPR-07) in four hybrid families.
A biparental codominant inheritance was previously reported in C. sempervirens for these isozymes, whereas the genetic control of the RAPD markers was unknown. The markers allowed identification of all the parents. Progeny had a single genetic pattern that was strictly identical to that of the father, C. dupreziana. These results confirm our hypothesis that pollen development in C. dupreziana is apomictic. This leads to the production of embryos that are genetically unrelated to the other seed components (maternal sporophyte and gametophyte). The results explain very well the previosly discovered significant anomalies in the reproductive structure of C. dupreziana. In this variant, viable pollen (male gametophyte) is diploid, embryos do not have the same allozymes as their mothers, and the endosperm (seed nutritive tissue) is not haploid, although in Gymnosperms it is derived solely from the female gametophyte.
In our tests another cypress species, C. sempervirens, resulted a surrogate mother for this embryogenic pollen. This is, to our knowledge, the first report of paternal apomixis in plants. Inbreeding in small size popolation of C. dupreziana (231 individuals) reduces fitness and suvival in the progeny and increases the risk of extintion. Paternal apomixis can be considered as a vegetative multiplication, which can overpass the inbreeding risks. Now our hypothesis is that this deviant reproductive pattern evolved in response to the reduction of the C. dupreziana population size.
See for details "Nature, where the work was published (5 July 2001)
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