biology, and ecology, as well as overlapping ranges character in cladistic terms) to the hypothetical
in western North America (Hart 1973, Scott & grouping of species within the genus Acipenser
Grossman 1973). Moreover, according to Artyuk- based on the species morphology, ecology, biogeog-
hin & Andronov (1990), some ecological and bio- raphy and possible area of origin (Artyukhin & An-
logical characteristics are common to these two spe- dronov 1990, Artyukhin 1994, 1995).
cies and the Chinese sturgeon,A. sinensis.All three Among fishes, ananalogous mode of speciation
species areoctoploid (Table 1). Possibly, tetraploi- through multiple independent tetraploidization
dization occurred in the ancestor of all three spe- events is characteristic of only one group of teleosts.
cies. If the difference between the karyotype ofA. the family Cyprinidae (review in Buth et al. 1991).A
sinensisandA. transmontanus (8n = 264 ± 3 and 230 tetraploid ancestor of two species, Cyprinus carpio
± or 248 ± 8, respectively is real, then it means that andCarassius auratus,appeared through tetraploi-
at least two polyploidization events took place in dization about 16–20 million years ago(Risinger &
the ancestral 120-chromosome form. The next step Larhammar 1993, Larhammar & Risinger 1994).
of polyploidization which occurred in the 240-chro- Hexaploids in Eurasia and Africa (reviews in Vasi-
mosome ancestor of A. medirostris andA. mikadoi, liev 1985, Buth et al. 1991, Golubtsov & Krysanov
resulted in the formation of the genome of A. mika- 1993), as well as a l6/20n-ploid Asian species Dip-
doi(which is, therefore, the youngest in this group tychus dipogon(Yu & Yu 1990) also were formed
of species). The Amur River sturgeon,A. schren- within Cyprinidae. Moreover, as a result of a tetra-
ckii.also seems to be an octoploid (but these data ploidization event which occurred approximately
are preliminary, see Table 1), as the species of the 50 million years ago an ancestor of another family,
trans-Paci ficA. sinensis-A medirostris-A. trans- Catostomidae, appeared within this group (Uyeno
montanusgroup, and lives in a close geographic & Smith 1972). Catostomids are considered to have
area. According to Artyukhin (1994, 1995), A. been tetraploid since then (Ferris & Whitt 1979, Ue-
schrenckiiis closely related to the Ponto-Caspian no et al. 1988,Tsoi et al. 1989). Tetraploids were also
speciesA. nudiveritrisandA. ruthenus formed in another family closely related to Cyprin-
It is difficult to draw any conclusions concerning clae, the Cobitidae (review in Vasiliev 1985). Only
the ancestor of A. brevirostrum.the species with the one other group of teleosts, the Salmonidae (which
second highest level of DNA content. The other includes three subfamilies, Coregoninae, Thymalli-
species of the Eastern Coast of North America,A. nae, and Salmoninae, sensu Nelson 1994) also orig-
oxyrinchus,is closely related to the European 120- inated from a tetraploid ancestor (Cold 1979, Al-
chromosome Atlantic sturgeon,A. sturio,and, ac- lendorf & Thorgaard 1984). By contrast with the
cording to the DNA content, has the same ploidy acipenseriforms, cyprinids, and catostomids, a de-
(Table 1). Possibly, A. brevirostrumis also related to crease in chromosome number through centromer-
some European sturgeons (A.nidiventris; see be- ic fusion was characteristic for different lineages of
low), and originated from these European-related salmonids (Vasiliev 1985, Buth et al. 1991). There-
ancestors through polyploidization. Because of its lore, a gradual increase in chromosome number
high ploidy level,A.brevirostrummight be a young through polyoidization has occurred a few times
species among East American repesentatives of during the history ofactinopterygians.
the genusAcipenser.As for another American spe-
cies, the freshwater lake sturgeon, A. fulvescens,
which is also an octoploid likeA. oxyrinchus(ac-
cording to its DNA content), its origin and relation-
ships with the other species are still unclear, al-
though it is morhologically similar toA. breviros-
rum (Vladykov & Greeley 1963, Findeis 1993).
Therefore, the cytogenetic data provide additional
information (ploidy level can be considered as a
Molecular phylogeny of the AcipenseriformesOur first objective was to search for molecular syn-
apomorphies of Acipenseriformes and its major in-
cluded clades using representatives of all extant
genera (except the Chinese paddlefish, Psephurus
gladius,due to our inability to obtain suitable tis-