Chromosome polymorphism and complements in populations of Girardia species (Platyhelminthes, Tricladida, Paludicola) from southern Brazil

The karyotypes of four species of freshwater triclads of the genus Girardia (Platyhelminthes), i.e. G. schubarti, G. tigrina, G. anderlani, and G. biapertura, from populations of different localities of the Rio Grande do Sul State, in southern Brazil, were analyzed. The karyotype of G. biapertura is presented for the first time. Three basic complements of 4, 8, and 9 chromosomes were found. Diploids, triploids, or mixoploids (2n/3n) specimens were frequently detected in these populations. The basic chromosomal complement of n = 9 was verified in two different species (G. biapertura and G. anderlani), presenting a large acrocentric chromosome which is rare in the family Dugesiidae. An intra and interspecific chromosomal variability was also detected and its evolutionary implications are discussed.

Os cariótipos de quatro espécies de planárias de água doce do gênero Girardia (Platyhelminthes), a saber, G. schubarti, G. tigrina, G. anderlani e G. biapertura, de populações ocorrentes em diferentes locais do estado do Rio Grande do Sul, na região sul do Brasil, foram analisados. O cariótipo de G. biapertura é apresentado pela primeira vez. Foram observados três complementos básicos, de 4, 8 e 9 cromossomos. Espécimes diplóides, triplóides e mixoplóides (2n/3n) foram observados freqüentemente nessas populações. O complemento cromossômico básico de n = 9 foi verificado em duas espécies (G. biapertura e G. anderlani), apresentando um grande cromossomo acrocêntrico que é raro na família Dugesiidae. Também foi observada certa variabilidade cromossômica, tanto intra- como interespecífica, cujas implicações evolutivas são discutidas.

Codon usage and bias in mitochondrial genomes of parasitic platyhelminthes

Sequences of the complete protein-coding portions of the mitochondrial (mt) genome were analysed for 6 species of cestodes (including hydatid tapeworms and the pork tapeworm) and 5 species of trematodes (blood flukes and liver- and lung-flukes). A near-complete sequence was also available for an additional trematode (the blood fluke Schistosoma malayensis). All of these parasites belong to a large flatworm taxon named the Neodermata. Considerable variation was found in the base composition of the protein-coding genes among these neodermatans. This variation was reflected in statistically-significant differences in numbers of each inferred amino acid between many pairs of species. Both convergence and divergence in nucleotide, and hence amino acid, composition was noted among groups within the Neodermata. Considerable variation in skew (unequal representation of complementary bases on the same strand) was found among the species studied. A pattern is thus emerging of diversity in the mt genome in neodermatans that may cast light on evolution of mt genomes generally.