Rampin, Massimiliano (2008) Evoluzione cariologica e citotassonomia in anfipodi. [Ph.D. thesis]
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Cytogenetics may contribute with non conventional characters to the phylogenetic reconstruction and to identify peculiar evolutionary paths. The order Amphipoda (Crustacea) is almost cytogentically unexplored: out of 8000 described species karyological knowledge concerns chromosome number for a hundred of them and the use of the molecular cytogenetics was limited to only two species.
During the PhD course, karyological investigations have been carried out for about forty amphipod species and the following parameters were analyzed: chromosome number, karyotype morphology, distribution and composition of constitutive heterochromatin, location in the chromosomes of specific repeated genomic sequences (ribosomal genes, telomeric sequences ), and nuclear DNA amount. The contribution of present thesis to the cytogenetical knowledge on amphipods is summarized in the following table.
parameter total of known species herein studied species contribution %
Chromosome number 130 27 21
Karyotype formula 47 27 57
Nucler DNA content 41 24 59
18S rDNA 31 30 97
5S rDNA 4 4 100
Telomeric sequence 16 15 94
C-banding 9 8 89
A wide variability in chromosome number characterizes the families Aoridae, Gammaridae and Ischyroceridae, whereas such parameter is relatively constant in the families Talitridae, Niphargidae, Gammaridae and Caprellidae. Within Amphipoda the lowest chromosome numbers were found in Aoridae and Ischyroceridae.
Amphipod karyotypes are generally characterized by a high symmetry due to the preponderant presence of median centromere chromosomes. Due to their relative rarity, the possible presence of sub-telocentric and/or acrocentric chromosomes may be taken into account as a diagnostic tool for related species.
Fluorescent in situ hybridization allowed to locate on the chromosomes the major complex of ribosomal genes. This gene cluster is more frequently located in terminal position, in a single homologous pair, but there are species where NORs involve more homologous pairs.
Different degrees of polymorphism may interest the NORs, both in dimensional and numerical terms. Being the most frequent condition, a single pair of NOR bearing chromosomes is supposed as the ancestral condition for amphidods, as well as it was assumed for vertebrates and mollusks.
The attempts to locate the minor complex of ribosomal genes (5S rDNA) gave positive results only for four species.
The chromosome telomeres of all the analyzed amphipods are characterized by the repeated pentameric sequence TTAGG. This sequence is presumed as the cytogenetical plesiomorphic character for the arthropods and was derived from the hexamer TTAGGG, present in most of invertebrates and in all vertebrates.
The data on the nuclear DNA content pointed out a wide variability of such parameter at family level, and the absence of a direct correlation with chromosome number, also in species for which an increase of chromosome number by polyploidization was supposed.
The increase of Genome Size may be a pre-adaptive character in those species living under extreme conditions like, for instance, those colonizing terrestrial or semi-terrestrial environments or those distributed in the highest latitude geographical areas.
A comparative analysis of the results for some families and/or super-families was carried out. The possible role of polyploidization to justify largely different chromosome numbers was considered. In the families Ischyroceridae and Caprellidae a correlation between number of NOR elements and chromosome number was found, this fact would make reasonable an evolution by polyploidization.
Finally, on a cytogenetical base it was possible to identify a new specie of Jassa, J. cadetta, in the Venice Lagoon. This new species is morphologically close to J. marmorata, and its differentiation was also enhanced by the comparison of species-specific profiles given by RAPD-PCR.
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