Abstract
An understanding of the nature of the chromosomes of the filariae is expected to greatly assist the future interpretation of genome data. Filarial development is not eutelic, and there does not seem to be a fixed number of cell divisions in the way that there is in Caenorhabditis. It is not clear whether the chromosomes of the filariae have localized centromeres or whether they are holocentric. Sex determination is by a chromosomal "balance" X0 system in most filariae, but in some Onchocercidae there has been a chromosomal fusion to create a neo-XY system. It is presumed that the molecular basis of sex determination in filariae is similar to Caenorhabditis. The ancestral karyotype of the filariae is probably 5A+X0, but in some Onchocercidae this has been reduced to 4A+XY, and in O. volvulus and O. gibsoni it has been further reduced to 3A+XY. Onchocerca volvulus and O. gibsoni both have supernumary (B-) chromosomes and in O. volvulus there is a single active nucleolus organising region near the middle of the long autosome.
Highlights
Filariae and other nematodes have small genomes relative to other multicellular eukaryotes
The interpretation of the available nuclear genome sequence of Brugia malayi and the expressed sequence tag (EST) libraries for Onchocerca volvulus, Onchocerca ochengi, Wuchereria bancrofti, Brugia malayi, Dirofilaria immitis and Litomosoides sigmodontis [3] as well as any possible O. volvulus genome sequencing project will be helped by an understanding of their chromosomes
This was obvious for the X chromosome pair because a more normal side by side pairing was observed in only 10% of ova
Summary
Filariae and other nematodes have small genomes relative to other multicellular eukaryotes. Bain [30] held that O. volvulus was taxonomically more closely related to O. ochengi, and recent molecular phylogeny reconstruction using the 12s, 16s and ND5 mitochondrial genes very strongly supports this view (Morales Hojas, Cheke and Post, unpublished data) If this is true, it would indicate that the similar karyotypes of O. volvulus and O. gibsoni have been produced by two different fusion events (which may not even have involved the same pairs of autosomes), and this is likely to be reflected in the precise position of the translocation breakpoints at the molecular level. If the origin of http://www.filariajournal.com/content/4/1/10 the 3A+XY karyotype was the result of some sort of autosome-autosome fusion this would result in one new large autosome (see Figure 2), and possibly a small chromosomal fragment [27]. There may be other inactive clusters of rDNA elsewhere in the genome, and it is interesting to note that rDNA has sometimes been implicated at the sites of chromosome mutations (such as translocations) in other organisms such as humans [33]
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