278
E. Pisano et al.
An alternative, less parsimonious, hypothesis is that pericentric
inversions are a frequent mutation which became' independently fixed in
various phylogenetic lines. The fact that two apparently similar small metasubmetacentric elements are present in other Antarctic perciforms [4] gives
some support to the second hypothesis. However, until chromosomal
homologies can be established by molecular methods neither of these two
hypotheses can be tested.
Nototheniidae and Bathydraconidae appear to be the most heterogeneous
families (Fig. 2, Table 1). Three main pathways in chromosome
differentiation were found in Nototheniidae. In one, the species have not
diverged very much from the ancestral karyotype and has maintained the
same diploid number (Fig. 2,4a). With the exception of the more derived
Pleuragramminae, at least one species in each nototheniid subfamily
conserves this pattern, including species of the genera Dissostichus,
Trematomus, Pagothenia, Lepidonotothen, Patagonotothen. In the second
lineage (Fig. 2,4b), additional rearrangements apparently gave rise to a
higher chromosomal number, as in Trematomus nicolai [12]. The third
group of species has highly rearranged karyotypes with a reduced number
of large metacentric elements (Fig. 2,4c), as it is found in some
Trematomus and typically in Notothenia and Paranotothenia species [2].
The karyotypic diversity found in Nototheniidae is not surprising
considering that they are the most diverse family in the suborder in terms of
species number, geographic distribution and in ecological characteristics [13,14].
The seven bathydraconid species studied so far have notably different
karyotypes [2,10]. Here the main three karyotypic lines are illustrated.
One is represented only by the species Psilodraco breviceps and
apparently has retained a primitive condition of 48 acrocentric
chromosomes (Fig. 2,7a). However, further study and banding is needed to
confirm if its karyotype is really plesiomorphic.
A slightly divergent set of chromosomes characterizes a second
karyotypic line in dragon fish (Fig. 2, 7b). The more derived line leads to a
symmetrical karyotype made of all metacentric elements (Fig. 2,7c), as
found in Prionodraco evansii.
The presence of intraspecific sex-linked heteromorphism is an additional
feature of high karyotypic variability In both Nototheniidae and
Bathydraconidae (Table 1).
The high karyotypic diversity found in both Nototheniidae and
Bathydraconidae is consistent with various molecular and morphological
results suggesting the paraphyletic nature of these two notothenioid
families (Bargelloni and Lecointre, this Vol.).
In contrast, the family Channichthyidae has very homogeneous
karyotypes. All the 13 studied species have 47-48 chromosomes which
have apparently diverged little from the supposed ancestral condition (Fig.
E. Pisano et al.
An alternative, less parsimonious, hypothesis is that pericentric
inversions are a frequent mutation which became' independently fixed in
various phylogenetic lines. The fact that two apparently similar small metasubmetacentric elements are present in other Antarctic perciforms [4] gives
some support to the second hypothesis. However, until chromosomal
homologies can be established by molecular methods neither of these two
hypotheses can be tested.
Nototheniidae and Bathydraconidae appear to be the most heterogeneous
families (Fig. 2, Table 1). Three main pathways in chromosome
differentiation were found in Nototheniidae. In one, the species have not
diverged very much from the ancestral karyotype and has maintained the
same diploid number (Fig. 2,4a). With the exception of the more derived
Pleuragramminae, at least one species in each nototheniid subfamily
conserves this pattern, including species of the genera Dissostichus,
Trematomus, Pagothenia, Lepidonotothen, Patagonotothen. In the second
lineage (Fig. 2,4b), additional rearrangements apparently gave rise to a
higher chromosomal number, as in Trematomus nicolai [12]. The third
group of species has highly rearranged karyotypes with a reduced number
of large metacentric elements (Fig. 2,4c), as it is found in some
Trematomus and typically in Notothenia and Paranotothenia species [2].
The karyotypic diversity found in Nototheniidae is not surprising
considering that they are the most diverse family in the suborder in terms of
species number, geographic distribution and in ecological characteristics [13,14].
The seven bathydraconid species studied so far have notably different
karyotypes [2,10]. Here the main three karyotypic lines are illustrated.
One is represented only by the species Psilodraco breviceps and
apparently has retained a primitive condition of 48 acrocentric
chromosomes (Fig. 2,7a). However, further study and banding is needed to
confirm if its karyotype is really plesiomorphic.
A slightly divergent set of chromosomes characterizes a second
karyotypic line in dragon fish (Fig. 2, 7b). The more derived line leads to a
symmetrical karyotype made of all metacentric elements (Fig. 2,7c), as
found in Prionodraco evansii.
The presence of intraspecific sex-linked heteromorphism is an additional
feature of high karyotypic variability In both Nototheniidae and
Bathydraconidae (Table 1).
The high karyotypic diversity found in both Nototheniidae and
Bathydraconidae is consistent with various molecular and morphological
results suggesting the paraphyletic nature of these two notothenioid
families (Bargelloni and Lecointre, this Vol.).
In contrast, the family Channichthyidae has very homogeneous
karyotypes. All the 13 studied species have 47-48 chromosomes which
have apparently diverged little from the supposed ancestral condition (Fig.