Due to the recent evolution of sequencing techniques, the number of available
genomes is rising steadily, leading to the possibility to make large scale
genomic comparison between sets of close species. An interesting question to
answer is: what is the common functionality genes of a collection of species,
or conversely, to determine what is specific to a given species when compared
to other ones belonging in the same genus, family, etc. Investigating such
problem means to find both core and pan genomes of a collection of species,
\textit{i.e.}, genes in common to all the species vs. the set of all genes in
all species under consideration. However, obtaining trustworthy core and pan
genomes is not an easy task, leading to a large amount of computation, and
requiring a rigorous methodology. Surprisingly, as far as we know, this
methodology in finding core and pan genomes has not really been deeply
investigated. This research work tries to fill this gap by focusing only on
chloroplastic genomes, whose reasonable sizes allow a deep study. To achieve
this goal, a collection of 99 chloroplasts are considered in this article. Two
methodologies have been investigated, respectively based on sequence
similarities and genes names taken from annotation tools. The obtained results
will finally be evaluated in terms of biological relevance