iii
Abstract
Arbuscular Mycorrhizal Fungi (AMF) have a particular and complex genetic
structure. Yet, many aspects of their genetics are still misunderstood and poorly
documented. These organisms reproduce by asexual multinucleate spores. In this study, I
investigated the mechanisms of genetic inheritance of nuclei through asexual reproduction
in AMF. First, I determined the number of nuclei in mature and juveniles spores; I used
statistical analysis to determine the relationship between the number of nuclei and the spore
diameter. Four species from the genus Glomus were observed with a confocal microscope.
The results showed that the number of nuclei has a significant positive relationship with
spore diameter and more importantly, surprising heterogeneity in the number of nuclei
among sister spores was found. To determine the number of nuclei in extraradical phases,
three different structures from the mycelia were carefully examined with a confocal
microscope. All the structures possessed more than one nucleus and showed that AMF
probably lack a single-nucleus stage during their extraradical phases. To study the nuclei’s
heritance, two different approaches were used: (i) Glomus irregulare was grown on
medium complemented with aphidicolin to inhibit the mitosis. Observations with a
confocal microscope permit to count the nuclei that come from the hyphae and not from the
mitosis. The results showed that massive nuclear migration and mitosis are the mechanisms
by which AMF spores are formed. (ii) The second approach confirm these results because
with time-laps live cellular imaging of young spores of Glomus diaphanum it was possible
to see many nuclei to get in the spores.
In a second part of this thesis, I studied horizontal gene exchanges among AMF
isolates through anastomoses. Thus, four isolates of the species G. irregulare were used in
in vitro crossing experiments, in total six combinations using two isolates per crossing
experiment. These crossing co-cultures were maintained over two years by subculturing.
Spores of two different isolates were confronted in vitro prior to observation of
anastomoses. 13% of spores formed anastomoses suggesting the occurrence of genetic
exchange between two isolates. A mitochondrial molecular marker referred as Indel 5, was