Eukaryotes Genomes - GUILLARDIA THETA

Guillardia theta (formerly known as Cryptomonas phi) is a cryptomonad alga (the common name for a group of unicellular life forms), it is an example of a cell-within-a-cell, being composed of a flagellate host cell, complete with mitochondria and nucleus, surrounding a plastid lying within a reduced cytoplasmic compartment that contains a vestigial nucleus (or nucleomorph). It is organised on 3 chromosomes and has one of the smallest nuclei sequenced to date - 0.55 million base pairs.

Guillardia theta has unusual and complex evolutionary origins, its micronucleus (nucleomorph) is the result of an ancient acquisition. Hundreds of millions of years ago, Guillardia theta came into being when a eukaryotic cell incorporated a red algae, a primitive plant cell which itself had formed when a eukaryotic cell incorporated a cyanobacterium. Because cyanobacteria can make food from sunlight, so did the red algae. In its new home, the cyanobacterium settled down to became a chloroplast, while the red algae's nucleus was distilled down to form a "nucleomorph".

Nucleomorphs are the highly reduced nuclei of 'endosymbiotic' algal cells that, in the distant past, set up home within unicellular hosts to mutual benefit. Like their hosts, the endosymbionts were eukaryotic, meaning, loosely, that they had a nucleus. Importantly, they were also photosynthetic, feeding their hosts with the products of this chemical reaction - carbohydrates and oxygen.

Endosymbioses is widespread however cryptomonad endosymbiosis belongs to a special category known as secondary endosymbiosis, whereby the photosynthetic captive becomes an integral, enduring part of the host cell. In the case of G. theta , the endosymbiont became a complex chloroplast. Over time - perhaps as many as 600 million years - the nucleus of this endosymbiont lost most of its genes. however research shows that the nucleomorph, is a bona fide nucleus. It has the usual eukaryotic trappings: several linear chromosomes; introns, possible centromeres and histones. Indeed, the nucleomorph is a fairly typical nucleus except for two features - an impoverished complement of genes, and an almost complete lack of non-coding DNA.

Evolutionary forces that shape genome size are not well understood but it is believed that streamlining of the nucleomorph genome is unlikely to be driven solely by natural selection for minimal DNA content, rather, it may be a result of uncontrolled DNA loss, and it is here that comparison of this and the human genome is likely to provide great interest to resaerechers.

Nucleomorphs do not have sex chromosomes and each of the three chromosomes is thought to be paired. Nevertheless, nucleomorph chromosomes are probably denied the normal opportunity to recombine because cryptomonad endosymbionts do not seem to undergo meiosis followed by genetic exchange with other nucleomorphs.

Scientists are now concentrating on determining not how the nucleomorph genome became so small, but why it stopped reducing where it did. This applies not only to cryptomonads but also to chlorarachniophyte algae, as their genomes have condensed to a similar end-point.

Gene sequences from the G. theta nucleomorph indicate that, as in other eukaryotes, the DNA is wrapped around histone proteins, forming 'chromatin' . However, in contrast to other eukaryotes, nucleomorph chromatin apparently does not condense into higher-order structures during cell division. Researchers have calculated that uncondensed nucleomorph chromosomes are only just short enough to fit inside a nucleomorph. If the nucleomorph DNA were packaged into fewer than three chromosomes, then those chromosomes would be too large to segregate during cell division. Conversely, if the DNA were separated onto more than three chromosomes, they might be too small to survive.

Nucleomorphs are kept only while they encode something necessary for survival, probably proteins required to operate and maintain the chloroplast. Most phytoplankton - the algal backbone of aquatic food chains - also acquired their chloroplasts in the same way as cryptomonads however in these phytoplankton, all genes have been transferred to the host nucleus from the engulfed nuclei, which has been lost.

References:

Nucleic Acids Res. 18(7):1903-1903(1990)
http://bric.postech.ac.kr/science/97now/01_4now/010426d.html
http://www.mbl.edu/CASSLS/DOUGLAS.ABS.html