Phylogenetic Analysis of 18S rDNA data from species of Pediastrum (Tracy Carlson, 2002):

Abstract (Regional Tri-Beta Conference, April, 2002):

Genus Pediastrum Found to be Non-Monophyletic
Tracy Carlson

The genus Pediastrum possesses uniquely sculptured, double-layered cell walls. Pediastrum has been the focus of much research because of these polymer-reinforced cell walls, the strength of which makes it one of the few freshwater algae to exist in the fossil record. Although algae are traditionally classified by morphology, molecular biology now allows for analysis of their genetic similarities. This study, encompassing species of Pediastrum and related genera Tetraedron and Sorastrum, represents the first genetic evolutionary comparison of these algae. The 18S rDNA, or small ribosomal subunit gene, was chosen for this investigation because it is variable enough to expect differences among species. Cultures were grown and harvested, breaking cells both chemically and mechanically. DNA was then extracted using a series of phenol/chloroform treatments. After purification, the target gene was amplified using PCR. Automated sequencing was performed by the University of Tulsa's Comparative Genomics Center. The best evolutionary tree was inferred by comparison of the sequences using the parsimony method. Surprisingly, the data show that Pediastrum appears in at least two distinct lineages and therefore is not a monophyletic group. One lineage, which includes P. angulosum, P. boryanum v. cornutum, and P. duplex, also contains a colonial alga, Hydrodictyon. The other branch places P. boryanum v. longicorne sister to Sorastrum sp. These findings contradict the belief that Pediastrum represents a single evolutionary branch. If well-supported, this new data will impact the taxonomy of these genera and will change the view of evolution within the group.

Phylogram from parsimony analysis of 18S rDNA sequence data using PAUP* 4.0b10 (Swofford 2001). Ingroup sampling included Pediastrum spp. and other sphaeroplealean taxa. The tree was rooted using data from other chlorophycean taxa. Bootstrap values ( greater than or equal to 50%) from 1000 replicates are listed at the appropriate internode. Branch lengths are drawn proportionally to the inferred change along the branch.