Coleman AW

Comparison of Eudorina/Pleodorina ITS sequences of isolates from nature with those from experimental hybrids
AM J BOT 89 (9): 1523-1530 SEP 2002

Abstract:
Internal transcribed spacer (ITS) regions of nuclear ribosomal repeats were compared among 50 Eudorina and Pleodorina isolates and two Volvox species known to clade with Eudorina species. Of the six major subclades found, four containing Eudorina and Pleodorina illinoisensis isolates, one containing Eudorina and Pleodorina indica, and one containing Volvox gigas and V. powersii, the basal branching order remains uncertain, but the positioning of isolates known to mate was always as nearest neighbors on the terminal branches of the tree. Four hybrid clones from a cross of E. elegans with P. illinoisensis, known from chromosome counts to be products of the failure of meiosis at zygote germination, contain both parental ITS repeat regions, as expected. However, they have in addition both crossover and other variant ITS cistrons among their many repeats of ITS. Such variation is limited to terminal regions of helices, as recognized from knowledge of RNA transcript secondary structure. Proper alignment then utilizes all of the nucleotide positions; the hybrid variants appear in positions intermediate between their parents in the tree. In fact, such variants seem to be hallmarks of recent hybridization events, since they were not found in any of the other 50 isolates.

Nozaki H, Takahara M, Nakazawa A, et al.

Evolution of rbcL group IA introns and intron open reading frames within the colonial Volvocales (Chlorophyceae)
MOL PHYLOGENET EVOL 23 (3): 326-338 JUN 2002

Abstract:
Mobile group I introns sometimes contain an open reading frame (ORE) possibly encoding a site-specific DNA endonuclease. However, previous phylogenetic studies have not clearly deduced the evolutionary roles of the group I intron ORFs. In this paper, we examined the phylogeny of group IA2 introns inserted in the position identical to that of the chloroplast-encoded rbcL coding region (rbcL-462 introns) and their ORFs from 13 strains of five genera (Volvox, Pleodorina, Volvulina, Astrephomene, and Gonium) of the colonial Volvocales (Chlorophyceae) and a related unicellular green alga, Vitreochlamys. The rbcL-462 introns contained an intact or degenerate ORE of various sizes except for the Gonium multicoccum rbcL-462 intron. Partial amino acid sequences of some rbcL-462 intron ORFs exhibited possible homology to the endo/excinuclease amino acid terminal domain. The distribution of the rbcL-462 introns is sporadic in the phylogenetic trees of the colonial Volvocales based on the five chloroplast exon sequences (6021 bp). Phylogenetic analyses of the conserved intron sequences resolved that the G. multicoccum rbcL-462 intron had a phylogenetic position separate from those of other colonial volvocalean rbcL-462 introns, indicating the recent horizontal transmission of the intron in the G. multicoccum lineage. However, the combined data set from conserved intron sequences and ORFs from most of the rbcL-462 introns resolved robust phylogenetic relationships of the introns that were consistent with those of the host organisms. Therefore, most of the extant rbcL-462 introns may have been vertically inherited from the common ancestor of their host organisms, whereas such introns may have been lost in other lineages during evolution of the colonial Volvocales. In addition, apparently higher synonymous substitutions than nonsynonymous substitutions in the rbcL-462 intron ORFs indicated that the ORFs might evolve under functional constraint, which could result in homing of the rbcL-462 intron in cases of spontaneous intron loss. On the other hand, the presence of intact to largely degenerate ORFs of the rbcL-462 introns within the three isolates of Gonium viridistellatum and the rare occurrence of the ORF-lacking rbcL-462 intron suggested that the ORF's might degenerate to result in the spontaneous intron loss during a very short evolutionary time following the loss of the ORF function. Thus, the sporadic distribution of the rbcL-462 introns within the colonial Volvocales can be largely explained by an equilibrium between maintenance of the introns by the intron ORF and spontaneous loss of introns when the introns do not have a functional ORE. (C) 2002 Elsevier Science (USA). All rights reserved.