Comparative Biochemistry and Physiology Part B 133 (2002) 155–161 1096-4959/02/$ - see front matter  2002 Elsevier Science Inc. All rights reserved. PII:S1096-4959 Ž 02 . 00104-5 Teleost introns are characterized by a high AqT content Paul Winnard, Bruce D. Sidell, Michael E. Vayda* Department of Biochemistry, Microbiology, and Molecular Biology and School of Marine Sciences, University of Maine, Orono, ME 04469-5735, USA Received 25 January 2002; received in revised form 15 May 2002; accepted 30 May 2002 Abstract We previously observed that Antarctic fish genes contain intron sequences of high AqT content (60–70% average AqT) which are in stark contrast with adjacent protein coding-sequences. Here, we report that this disparity in introny exon base composition is a common feature among teleosts. We analyzed 483 teleost genomic DNA sequences, containing 2583 introns, from 80 teleost genera that populate polar, temperate, or tropical habitats. Eighty-nine percent of teleost introns display an AqT content between 50–84% AqT with a mean of 60% AqT. In contrast, only 37% of teleost exons have an AqT content greater-than 50% with a mean of 48% AqT. A comparison to homologous mammalian genes showed a striking difference; in this case, introns and exons have similar base compositions, averaging 45–47% AqT. This indicates that most teleost genes exhibit a large difference in base composition between their introns and exons. There was no correlation of teleost intron AqT content to intron length or habitat temperature range. Thus, teleost intron sequences tend to show the common feature of being much higher in AqT content then neighboring exons.  2002 Elsevier Science Inc. All rights reserved. Keywords: Teleost; AqT content; Introns; Exons; Body temperature; Base composition; Antarctic icefish 1. Introduction Previous work characterizing gene structure of myoglobin and fatty acid-binding protein (FABP) from Antarctic teleost fish (family, Channichthyi- dae) indicated that the introns of these genes have base compositions of up to 71% AqT (Small et al., 1998). This is in comparison to average base compositions of only 49% AqT for introns of the homologous mammalian genes. Similarly, kinesin, dynein (H. William Detrich III, personal commu- nication) a-tubulin (GenBank accession no. AF082027), d-9-desaturase (GenBank accession no. AJ249579), and a-globin and b-globin genes (Zhao et al., 1998; GenBank accession no. *Corresponding author. Tel.: q1-207-581-2821; fax: q1- 207-581-2801. E-mail address: vayda@maine.maine.edu (M.E. Vayda). AF049916) from Antarctic nototheniids exhibit introns with high AqT contents. These fishes inhabit waters that are chronically cold. Channic- thyid and nototheniid teleosts are endemic to Antarctic coastal waters where year-round temper- ature in McMurdo Sound is y1.86 8C and along the Antarctic Peninsula, where temperature fluc- tuates from y1.86 to q1.0 8C over the course of the year (Eastman, 1993). Small et al. (1998) hypothesized that the high AqT content of Ant- arctic teleost introns may be an adaptation to overcome thermal barriers that might affect effi- cient metabolism of DNA or RNA at low temperatures. The base composition of genomic sequences has been reported to affect the thermal stability of nucleic acid structures (Wada and Suyama, 1986a,b). Thus, base composition and temperature