rent views on paleogeography will provide useful The size of macrochromosomes in both groups is
tools for resolving complex relationships and phy- between 2–5 μm, and the majority of macrohromo-
logeny of sturgeons’. In the first part of this paper somes are the meta-and submetacentrics (Table 2).
we review all cytogenetic data available on Acipen- One third to one half of the chromosome number in
seriformes and make some new conclusions rele- both groups is comprised of microchromosomes of
vant to the four groups withinAcipensermentioned a very small size (about 1 μm). Karyotypes of the
above. In the second part we describe experimental 120 - chromosome species typically consist of 4 pairs
data on the molecular phylogeny of Acipenseri- of large metacentrics (no. 1–4), 5 pairs of large but
formes. Because multiple gene regions have been somewhat smaller metacentrics (no. 5–9), about 20
useful in other groups of fishes (reviews in Normark pairs of medium-sized metacentrics and/or sub-
et al. 1991, Stock et al. 1991a, Meyer 1993, Patterson metacentrics of gradually decreasing size (no. 10–
et al. 1993), we believed that they might also provide 30). one pair of comparatively large telocentrics
reasonable character state information for acipen- (no. 30), one pair of small telocentrics, and approxi-
seriforms. Consequently we amplified and se- mately 56 ± 4 microchromosomes of different form
quenced partial fragments of 18S rRNA, 12S rRNA, (Table 2). The difference between karyotypes of
16s rRNA, and cytochromebgenes of a few repre- representatives of two lineages of the extant aci-
sentatives of all lineages of this order. We included penseriforms, the Polyodontidae (Polyodon spath-
the phylogenetic analysis of the combined molecul- ula)and Acipenseridae (all other 120-chromosome
ar and morphological data for all the species we species in Table 1), as well as of the lineages within
studied. Also, we examined relationships among the Acipenseridae (Huso huso, the 120-chromo-
representatives of four species groups of the genus some species of the genusAcipenser, andScaphir-
Acipenser recognized by Artyukhin (1995), using hynchus platorynchus), seems to be small. Evident-
data for a partial sequence of the cytochrome b ly, the ancestral acipenseriform karyotype was pre-
gene. served in these fishes without dramatic changes
during diverisification of the group.
In general, few karyotypic changes are noticeable
among the species ofAcipenserwith 120-chromo-
somes (Table 2). The karyotype of Huso huso is
more symmetric than those of the 120-chromosome
species ofAcipenser, i.e., it contains more biarmed
chromosomes and fewer microchromosomes (see
Morescalchi 1973). Also, there is a small difference
among the 120-chromosome species in the size of a
pair of large telocentrics (no. 30): it is small inA.
sturio(Fontana & Colombo 1974) andA. nudiven-
tris(Arefjev 1983) and it is as large as pair no. 8 or 9
inHuso huso(Fontana & Colombo 1974, Birstein &
Vasiliev 1987, Arefjev 1989b) andA. ruthenus(Rab
1986, Birstein & Vasiliev 1987).
The similarity of karyotypes of the 120-chromo-
some acipenseriforms points to a generally slow
rate of karyological evolution. This correlates with
a slow rate of nuclear DNA evolution: practically all
genome fractions (both the repeated and unique se-
quences) are homologous inAcipenser ruthenus,A.
stellatus,A. gueldenstaedtii, andH. huso,and the
number of nucleotide substitutions in the first frac-
Acipenseriform cytogenetics: an overviewMain karyotypic characteristics, DNA content, and
polyploidy
Karyotypes of about half of all sturgeon species
have been described and the DNA content in most
sturgeon species and American paddlefish has been
measured (Table 1). Acipenseriform karyotypes in-
vestigated sofar have two particular characteristics:
(1) they are large; and (2) they consist of macro-and
microchromosomes. According to the number of
chromosomes (2n), the species can be divided into
two groups: those with about 120 chromosomes
(e.g.,Huso huso, H. dauricus,Acipenser ruthenus,
A. stellatus,A. nudiventris,A. sturio, andPolyodon
spathula), and those with 240 chromosomes (e.g.,A.
gueldenstaedtii,A. naccarii, A. baerii,A. schrenckii,
andA. transmontanus). By comparison to the 120-
chromosome species, the 240-chromosome species
are tetraploids.