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

Chlorarachniophyte algae contain a complex chloroplast derived from the endosymbiosis of a eukaryotic alga. The reduced nucleus of the endosymbiont, the nucleomorph, is located between the inner and outer pair of membranes surrounding the chloroplast. The nucleomorph of chlorarachniophytes has previously been demonstrated to contain at least three small linear chromosomes. Here we describe cloning the end of the smallest nucleomorph chromosome which is shown to carry a telomere consisting of a tandemly repeated 7 bp sequence, TCTAGGG. Using the telomere repeat as a probe, we show that nucleomorph telomeres display typical hetero-disperse size distribution. The nucleomorph is shown to contain only three chromosomes with a haploid genome size of just 380 kb. All six nucleomorph chromosome termini are identical with an rRNA cistron closely linked to the telomere. The nucleomorph chromosomes thus have relatively large inverted repeats at their ends. Chromosomes from the host nucleus are shown to have a different telomere repeat motif to that of the nucleomorph chromosomes.