de la structure à la croissance cellulaire - Université Bordeaux 1

110 M. Bourdon et al.
syncitial female gametophyte (Yadegari and Drews 2004), in anther tapetum
(D’Amato 1984) and seed endosperm (Berger 2003), while nuclear fusion may
also contribute to polyploidization of the chalazal domain of the endosperm in
Arabidopsis (Baroux et al. 2004). Endomitosis corresponds to a doubling of the
chromosome number in nucleus. Chromosomes double and condense, sister chromatids
separate normally, but return to the interphase state within an intact nuclear
envelope, thus generating nuclear endopolyploidy. Although endomitosis mainly
occurrs in animals, it is only rarely encountered in plants (D’Amato 1984).
Endoreduplication represents the most common mode of cell endopolyploidization
in plants and is estimated to occur in over 90% of Angiosperms (Nagl 1976; D’Amato
1984). This process is an endonuclear chromosome duplication, which occurs in the
absence of any obvious condensation and decondensation steps leading to the production
of chromosomes with 2n chromatids without any change in chromosome number
(Joubès and Chevalier 2000; Edgar and Orr-Weaver 2001). As a consequence,
hypertrophying nuclei arise from successive cycles of DNA replication without
segregation of sister chromatids, and in extreme cases “giant” polytene chromosomes
are generated as observed for embryo suspensor cells of Phaseolus species (D’Amato
1984) or giant hair cells of Bryonia anthers (Barlow 1975).
3.2 Occurrence of Endopolyploidization in Fruit Species
As stated by D’Amato (1984), endopolyploidy is “of such a widespread occurrence
in plants, that it can be regarded as the rule rather than the exception.” A recent
survey performed on several vegetative organs of 54 seed plant species different in
term of genome size and belonging to two Gymnosperm and 14 Angiosperm
families by Barow and Meister (2003) indicated that endopolyploidy occurred in
33 species taken from ten different families. Though absent in Gymnosperms,
the frequent occurrence of endopolyploidy within Angiosperms seems restricted,
however, to certain phylogenetic groups (Barow 2006).
Though largely documented in vegetative organs (Barow and Meister 2003), the
occurrence of endopolyploidy in reproductive organs, especially in fruit tissues, and
its extent in different species has gained little attention so far. Endopolyploidization
was found in ovular tissues (antipodal cells, synergids, endosperm and embryo
suspensor cells) and in anther cells (anther hair, glandular hair and anther tapetal
cells), where endoreduplication seems to be the preferential way of polyploidization
(D’Amato 1984; Carvalheira 2000; Bauer and Birchler 2006).
More than 60 years ago, endopolyploidization was reported initially in cucumber
and other fruit materials (cited in Coombe 1976 and Barow 2006) and then in the
mesocarp parenchyma cells in apricot (Bradley and Crane 1955). However, in this
latter report the natural and/or physiological occurrence of the phenomenon in this
fruit could be questioned as these authors observed polysomaty (increase in chromosome
number according to endomitosis), after an auxin treatment and subsequent