280
E. Pisano et al.
Pseudaphritis) leaving together in the family Bovichtidae the genera
Cotto perea and Boviehtus. Phylogenetic analysis of various data sets has
shown that the Bovichtidae are not monophyletic. The analysis of
hemoglobin sequence places Pseudaphritis closer to non-bovichtid
notothenioids [19] and analyses based on 28S rDNA and mitochondrial
DNA sequencing clearly separate Cottoperea-Boviehtus from the more
derived Pseudaphritis [20,21]. Cytogenetic evidence indicates different
pathways of karyological differentiation in Cotto perea gobio and
Pseudaphritis urvillii, a karyologically derived taxon [3]. Moreover, the
two pairs of small metacentric elements in the karyotype of Pseudaphritis
seem to indicate a common karyotypic pathway in this species and other
notothenioid phyletic lineages (Fig. 2). Karyotypic features of B.
angustifrons mark the taxonomic distance between Pseudaphritis and the
taxa Cotto perea and Boviehtus. Although cytogenetic data cannot help to
estimate divergence times, they strongly support the separation of the taxon
Pseudaphritidae. Because Cotto perea and Bovichtus both retain a primitive
karyological condition in chromosome number, morphology and banding
patterns, no reliable cytotaxonomic hypotheses can be developed for these
two taxa.
Intraspecific Polymorphism
As the amount of cytogenetic research on Antarctic fish increased and new
methods have been applied, more intraspecific polymorphisms of both
autosomes and sex-linked chromosomes have been found.
One of the most common source of intraspecific polymorphism seems to
be linked to the amount of ribosomal genes per haploid set, giving rise to
two homologous autosomes bearing different sized nucleolar organizing
regions (NORs) [8]. In this respect, the Antarctic species do not differ very
much from temperate and tropical fishes where intra- and interindividual
variation of these DNA regions has been widely reported [22].
Several species of channichthyids and nototheniids and one
bathydraconid species possess heteromorphic sex-linked chromosomes
with male heterogamety (Table 1). In most cases a multiple Y and an odd
(2n-l) number of chromosomes is present in the males. This feature seems
to have been both independently and recently acquired by notothenioid taxa
and it has been considered a convergent apomorphic character in
karyological differentiation of Notothenioidei [1,6,12].
Although sex chromosomes are present in some non-Antarctic perciform
species [5,23], the frequency of sex-linked heterochromosomes seems
higher among Antarctic fish. For this reason it has sometimes been
considered as an adaptation of sex-determining mechanisms to the
E. Pisano et al.
Pseudaphritis) leaving together in the family Bovichtidae the genera
Cotto perea and Boviehtus. Phylogenetic analysis of various data sets has
shown that the Bovichtidae are not monophyletic. The analysis of
hemoglobin sequence places Pseudaphritis closer to non-bovichtid
notothenioids [19] and analyses based on 28S rDNA and mitochondrial
DNA sequencing clearly separate Cottoperea-Boviehtus from the more
derived Pseudaphritis [20,21]. Cytogenetic evidence indicates different
pathways of karyological differentiation in Cotto perea gobio and
Pseudaphritis urvillii, a karyologically derived taxon [3]. Moreover, the
two pairs of small metacentric elements in the karyotype of Pseudaphritis
seem to indicate a common karyotypic pathway in this species and other
notothenioid phyletic lineages (Fig. 2). Karyotypic features of B.
angustifrons mark the taxonomic distance between Pseudaphritis and the
taxa Cotto perea and Boviehtus. Although cytogenetic data cannot help to
estimate divergence times, they strongly support the separation of the taxon
Pseudaphritidae. Because Cotto perea and Bovichtus both retain a primitive
karyological condition in chromosome number, morphology and banding
patterns, no reliable cytotaxonomic hypotheses can be developed for these
two taxa.
Intraspecific Polymorphism
As the amount of cytogenetic research on Antarctic fish increased and new
methods have been applied, more intraspecific polymorphisms of both
autosomes and sex-linked chromosomes have been found.
One of the most common source of intraspecific polymorphism seems to
be linked to the amount of ribosomal genes per haploid set, giving rise to
two homologous autosomes bearing different sized nucleolar organizing
regions (NORs) [8]. In this respect, the Antarctic species do not differ very
much from temperate and tropical fishes where intra- and interindividual
variation of these DNA regions has been widely reported [22].
Several species of channichthyids and nototheniids and one
bathydraconid species possess heteromorphic sex-linked chromosomes
with male heterogamety (Table 1). In most cases a multiple Y and an odd
(2n-l) number of chromosomes is present in the males. This feature seems
to have been both independently and recently acquired by notothenioid taxa
and it has been considered a convergent apomorphic character in
karyological differentiation of Notothenioidei [1,6,12].
Although sex chromosomes are present in some non-Antarctic perciform
species [5,23], the frequency of sex-linked heterochromosomes seems
higher among Antarctic fish. For this reason it has sometimes been
considered as an adaptation of sex-determining mechanisms to the