The chlorarachniophyte: a cell with two different nuclei and two different telomeres

Area of Science:

• * Molecular Biology
• * Algal Genetics
• * Endosymbiosis Studies

Background:

• * Chlorarachniophyte algae harbor a complex chloroplast originating from eukaryotic endosymbiosis.
• * A remnant nucleus, the nucleomorph, resides within the chloroplast, containing linear chromosomes.
• * Previous research indicated at least three small linear chromosomes within the nucleomorph.

Purpose of the Study:

• * To characterize the telomeres of chlorarachniophyte nucleomorph chromosomes.
• * To determine the number and genome size of nucleomorph chromosomes.
• * To investigate the structure of nucleomorph chromosome ends and compare them to host nuclear chromosomes.

Main Methods:

• * Cloning and sequencing of nucleomorph chromosome ends.
• * Hybridization using a telomere repeat probe.
• * Analysis of nucleomorph chromosome number and genome size.

Main Results:

• * Identified a novel telomere repeat sequence (TCTAGGG) on the smallest nucleomorph chromosome.
• * Confirmed the presence of only three linear chromosomes in the nucleomorph, with a haploid genome size of 380 kb.
• * Demonstrated that all six nucleomorph chromosome termini share identical telomere sequences and linked rRNA cistrons, forming large inverted repeats.
• * Revealed distinct telomere repeat motifs between nucleomorph and host nuclear chromosomes.

Conclusions:

• * Chlorarachniophyte nucleomorphs possess a compact genome organized into three linear chromosomes with conserved, large terminal inverted repeats.
• * The findings provide insights into the unique genome structure and evolution of endosymbiont-derived organelles.
• * Telomere analysis highlights differences between the nucleomorph and host nuclear genomes.