World Congress of Malacology Antwerp ... - Unitas Malacologica

will be decreased whereas polymorphism in nuclear DNA will be kept. The necessary condition of
"sweeping" in mtDNA is absence of recombination.
This hypothesis explains different locations of B. rugosa and B. dybowskiana on mtCO1 and nuclear
ITS1 phylogenetic and spanning trees.
Pronounced karyological divergence of the North American congeners Sphaerium
rhomboideum and S. occidentale (Bivalvia, Veneroida, Sphaeriidae)
Petkevičiūtė, Romualda 1 ; Stanevičiūtė, Grazina 1 ; Stunžėnas, Virmantas 1 ; Lee, Taehwan 2 ; Ó
Foighil, Dairmaid 2
1. Institute of Ecology, Vilnius University, Akademijos 2, LT-08412, Vilnius 21,
Lithuania,
Email: romualda@ekoi.lt; grasta@ekoi.lt; stunzen@ekoi.lt
2. Museum of Zoology and Department of Ecology and Evolutionary Biology, University of
Michigan, 1109 Geddes Avenue, Ann Arbor, MI 48109-1079, USA,
Email: taehwanl@umich.edu; diarmaid@umich.edu
Polyploid taxa pose significant challenges to systematists because they may be the products of
qualitatively distinct generative mechanisms (allo-/auto- polyploidization) that have important
implications for our ability to accurately infer genealogies. Although polyploidy is thought to be
uncommon in animals, recent chromosomal studies have revealed the presence of exceptionally
variable mitotic chromosome numbers, from 30 to 247, in the exclusively freshwater bivalve family
Sphaeriidae. These extraordinary karyological complements strongly suggest that pronounced
polyploidization is prevalent in this family and single-copy nuclear gene allelic trees of some North
American polyploid taxa have indicated an early genome duplication, predating the divergence of
three Sphaerium subgenera (Amesoda, Herringtonium, and Musculium). However, the evolutionary
origins of sphaeriid genome amplification remain obscure, as does the actual levels of ploidy they
exhibit.
Advancing our knowledge of sphaeriid genome amplification processes will hinge on the
identification of robust sister relationships among taxa differing in chromosome complements,
especially those involving diploid and polyploid species. In the present study, we karyotyped two
North American taxa, Sphaerium occidentale and S. rhomboideum, that displayed a robust and
exclusive sister relationship for a nuclear ribosomal marker (ITS-1) but, interestingly, were not sister
taxa for a mitochondrial ribosomal marker (16S). A chromosome count of ~209 has been recorded
for S. occidentale and our karyotype of this species is the first such for a polyploid sphaeriid.
Although a large fraction of chromosomes was too small to arrange with confidence, the largest and
medium-sized chromosomes of S. occidentale clustered into four subgroups on the basis of shared
size and morphology. S. rhomboideum’s chromosomal complement has not been previously studied
and we found it to be diploid (2n=44), the first such record for a New World sphaerinid and one that
represents a novel focal point for investigating the evolution of polyploidy in these taxa.
Evolution of mollusc lens crystallins
Piatigorsky, Joram
National Eye Institute, NIH, 7 Memorial Drive, Room 100, Bethesda, Maryland, USA,
Email: joramp@nei.nih.gov
The abundant lens crystallin proteins are diverse, multifunctional proteins that are often expressed
outside of the eye. Crystallin recruitment involved selective high lens expression with or without
gene duplication. S-crystallins, the major crystallins of cephalopods (squid, octopus), descended from
glutathione S-transferase (GST) and evolved by an initial gene duplication, elevated lens expression,
subsequent gene duplications, site-specific mutations and insertion of an additional exon. The single
squid GST gene is expressed principally in the digestive gland. The lens-preferred (if not specific)
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