Biological Sciences

 

Phylogenetic investigation on the order Ectocarpales (Phaeophyceae)

Researcher: Miss Marie Racault

Problematic:

Brown algae form the class Phaeophyceae which contains an extensive diversity of morphological forms, ranging from very small branched filaments or discs living as epiphytes or endophytes (see figures 1 and 2), to the giant kelps of more than 60 m length. The classification of the large order Ectocarpales (Phaeophyceae) has remained controversial over the years, mainly because it is based on few morphological/anatomical features. More recently, these views have been challenged by the developments in phylogenetic studies, as a result of DNA sequencing based on nuclear ribosomal and chloroplastic gene sequences.

The aim of this study is to contribute towards the elucidation of the molecular phylogeny of the order Ectocarpales, by inferring the systematic position of a number of small brown algal epiphytes.


For this purpose:

  • A number of small brown algal epiphytes from the North Atlantic are collected and identified (see example of epiphytes on figures 1 and 2).
Pustular appearance of Entonema sp. on its host Undaria pinnatifida
Hemispherical cushion of Elachista scutulata on Himanthallia sp.
Fig. 1. Pustular appearance of Entonema sp. on its host Undaria pinnatifida.
Fig. 2. Hemispherical cushion of Elachista scutulata on Himanthallia sp.
  • Sampled brown algae are isolated and cultivated in growth rooms under various photoperiod and temperature conditions, in order to study their morphology, cytology, development and life histories using microscopes (see morphological characteristics of some species on figures 3 to 6).
Squash Preparation (S.P.) of Myrionema papillosum fertile thallus
Portion of thallus of Petrospongium berkeleyi in S.P.
Fig. 3. Squash Preparation (S.P.) of Myrionema papillosum fertile thallus.
Fig. 4. Portion of thallus of  Petrospongium berkeleyi in S.P. stained with glycerol jelly

Compsonema microspongium unilocular sporangia in S.P.
Myrionema papillosum uniseriate plurilocular sporangia in S.P.
Fig. 5. Compsonema microspongium unilocular sporangia in S.P.
Fig. 6. Myrionema papillosum uniseriate plurilocular sporangia in S.P.
  • The large subunit and the spacer region of the Rubisco gene (see figure 9) within the chloropolast are sequenced in order to resolve the phylogenetic relationships amongst these small taxa.

Schematic representation of the Rubisco-operon in Phaeophyceae

Fig. 7. Schematic representation of the Rubisco-operon in Phaeophyceae and the expected length of the different amplicons. Primers positions are shown (provided as pairs).

  • Phylogenetic analyses are carried out using various models of evolution with the appropriate softwares (e.g. PAUP, Modeltest, treeview).