title: Spore development and nuclear inheritance in arbuscular mycorrhizal fungi.
abstract: BACKGROUND
  A conventional tenet of classical genetics is that progeny inherit half their genome from each parent in sexual reproduction instead of the complete genome transferred to each daughter during asexual reproduction. The transmission of hereditary characteristics from parents to their offspring is therefore predictable, although several exceptions are known. Heredity in microorganisms, however, can be very complex, and even unknown as is the case for coenocytic organisms such as Arbuscular Mycorrhizal Fungi (AMF). This group of fungi are plant-root symbionts, ubiquitous in most ecosystems, which reproduce asexually via multinucleate spores for which sexuality has not yet been observed.
  
  
  RESULTS
  We examined the number of nuclei per spore of four AMF taxa using high Z-resolution live confocal microscopy and found that the number of nuclei was correlated with spore diameter. We show that AMF have the ability, through the establishment of new symbioses, to pass hundreds of nuclei to subsequent generations of multinucleated spores. More importantly, we observed surprising heterogeneity in the number of nuclei among sister spores and show that massive nuclear migration and mitosis are the mechanisms by which AMF spores are formed. We followed spore development of Glomus irregulare from hyphal swelling to spore maturity and found that the spores reached mature size within 30 to 60 days, and that the number of nuclei per spores increased over time.
  
  
  CONCLUSIONS
  We conclude that the spores used for dispersal of AMF contain nuclei with two origins, those that migrate into the spore and those that arise by mitosis in the spore. Therefore, these spores do not represent a stage in the life cycle with a single nucleus, raising the possibility that AMF, unlike all other known eukaryotic organisms, lack the genetic bottleneck of a single-nucleus stage.
authors.affiliation: Université de Montréal, Département de sciences biologiques, Institut de recherche en biologie végétale, 4101 rue Sherbrooke Est, QC, H1X 2B2, Canada.
authors.forename: Julie
authors.initials: J
authors.lastname: Marleau
authors.forename: Yolande
authors.initials: Y
authors.lastname: Dalpé
authors.forename: Marc
authors.initials: M
authors.lastname: St-Arnaud
authors.forename: Mohamed
authors.initials: M
authors.lastname: Hijri
mesh_terms: D002467:Cell Nucleus / Q000235:genetics*; D018613:Microscopy, Confocal; D008938:Mitosis; D038821:Mycorrhizae / Q000166:cytology / Q000235:genetics*; D013172:Spores, Fungal / Q000166:cytology* / Q000235:genetics; D013997:Time Factors