Mitochondrial DNA of the coral sarcophyton glaucum contains a gene for a homologue of bacterial muts: A possible case of gene transfer from the nucleus to the mitochondrion

The nucleotide sequences of two segments of 6,737 ntp and 258 ntp of the 18.4-kb circular mitochondrial (mt) DNA molecule of the soft coral Sarcophyton glaucum (phylum Cnidaria, class Anthozoa, subclass Octocorallia, order Alcyonacea) have been determined. The larger segment contains the 3′ 191 ntp of the gene for subunit 1 of the respiratory chain NADH dehydrogenase (ND1), complete genes for cytochrome b (Cyt b), ND6, ND3, ND4L, and a bacterial MutS homologue (MSH), and the 5′ terminal 1,124 ntp of the gene for the large subunit rRNA (l-rRNA). These genes are arranged in the order given and all are transcribed from the same strand of the molecule. The smaller segment contains the 3′ terminal 134 ntp of the ND4 gene and a complete tRNA^f-Met gene, and these genes are transcribed in opposite directions. As in the hexacorallian anthozoan, Metridium senile, the mt-genetic code of S. glaucum is near standard: that is, in contrast to the situation in mt-genetic codes of other invertebrate phyla, AGA and AGG specify arginine, and ATA specifies isoleucine. However, as appears to be universal for metazoan mt-genetic codes, TGA specifies tryptophan rather than termination. Also, as in M. senile the mt-tRNA^f-Met gene has primary and secondary structural features resembling those of Escherichia coli initiator tRNA, including standard dihydrouridine and TψC loop sequences, and a mismatched nucleotide pair at the top of the amino-acyl stem. The presence of a mutS gene homologue, which has not been reported to occur in any other known mtDNA, suggests that there is mismatch repair activity in S. glaucum mitochondria. In support of this, phylogenetic analysis of MutS family protein sequences indicates that the S. glaucum mtMSH protein is more closely related to the nuclear DNA-encoded mitochondrial mismatch repair protein (MSH1) of the yeast Saccharomyces cerevisiae than to eukaryotic homologues involved in nuclear function, or to bacterial homologues. Regarding the possible origin of the S. glaucum mtMSH gene, the phylogenetic analysis results, together with comparative base composition considerations, and the absence of an MSH gene in any other known mtDNA best support the hypothesis that S. glaucum mtDNA acquired the mtMSH gene from nuclear DNA early in the evolution of octocorals. The presence of mismatch repair activity in S. glaucum mitochondria might be expected to influence the rate of evolution of this organism's mtDNA. Received: 13 January 1997 / Accepted: 23 September 1997     

Evolutionary Shifts in Three Major Structural Features of the Mitochondrial Genome Among Iguanian Lizards

A phylogenetic tree for major lineages of iguanian lizards is estimated from 1,488 aligned base positions (858 informative) of newly reported mitochondrial DNA sequences representing coding regions for eight tRNAs, ND2, and portions of ND1 and COI. Two well-supported groups are defined, the Acrodonta and the Iguanidae (sensu lato). This phylogenetic hypothesis is used to investigate evolutionary shifts in mitochondrial gene order, origin for light-strand replication, and secondary structure of tRNA^Cys. These three characters shift together on the branch leading to acrodont lizards. Plate tectonics and the fossil record indicate that these characters changed in the Jurassic. We propose that changes to the secondary structure of tRNA^Cys may destroy function of the origin for light-strand replication which, in turn, may facilitate shifts in gene order. Received: 28 May 1996 / Accepted: 27 December 1996     

Partial Sequence of a Sponge Mitochondrial Genome Reveals Sequence Similarity to Cnidaria in Cytochrome Oxidase Subunit II and the Large Ribosomal RNA Subunit

A 2550-bp portion of the mitochondrial genome of a Demosponge, genus Tetilla, was amplified from whole genomic DNA extract and sequenced. The sequence was found to code for the 3′ end of the 16S rRNA gene, cytochrome c oxidase subunit II, a lysine tRNA, ATPase subunit 8, and a 5′ portion of ATPase subunit 6. The Porifera cluster distinctly within the eumetazoan radiation, as a sister group to the Cnidaria. Also, the mitochondrial genetic code of this sponge is likely identical to that found in the Cnidaria. Both the full COII DNA and protein sequences and a portion of the 16S rRNA gene were found to possess a striking similarity to published Cnidarian mtDNA sequences, allying the Porifera more closely to the Cnidaria than to any other metazoan phylum. The gene arrangement, COII—tRNA^Lys—ATP8—ATP6, is observed in many Eumetazoan phyla and is apparently ancestral in the metazoa. Received: 24 November 1997 / Accepted: 14 September 1998     

Nucleotide Composition Bias Affects Amino Acid Content in Proteins Coded by Animal Mitochondria

We show that in animal mitochondria homologous genes that differ in guanine plus cytosine (G + C) content code for proteins differing in amino acid content in a manner that relates to the G + C content of the codons. DNA sequences were analyzed using square plots, a new method that combines graphical visualization and statistical analysis of compositional differences in both DNA and protein. Square plots divide codons into four groups based on first and second position A + T (adenine plus thymine) and G + C content and indicate differences in amino acid content when comparing sequences that differ in G + C content. When sequences are compared using these plots, the amino acid content is shown to correlate with the nucleotide bias of the genes. This amino acid effect is shown in all protein-coding genes in the mitochondrial genome, including cox I, cox II, and cyt b, mitochondrial genes which are commonly used for phylogenetic studies. Furthermore, nucleotide content differences are shown to affect the content of all amino acids with A + T- and G + C-rich codons. We speculate that phylogenetic analysis of genes so affected may tend erroneously to indicate relatedness (or lack thereof) based only on amino acid content. Received: 3 July 1996 / Accepted: 6 November 1996     

Mhc Allelic Diversity and Modern Human Origins

Thirty complete coding sequences of human major histocompatibility complex (Mhc) class II DRB alleles, spanning 237 codons, were analyzed for phylogenetic information using distance, parsimony, and likelihood approaches. Allelic genealogies derived from different parts of the coding sequence (exon 2, the 5′ and 3′ ends of exon 2, respectively, and exons 3–6) were compared. Contrary to prior assertions, a rigorous analysis of allelic genealogies in this gene family cannot be used to justify the claim that the lineage leading to modern humans contained on average at least 100,000 individuals. Phylogenetic inferences based upon the exon 2 region of the DRB loci are complicated by selection and recombination, so this part of the gene does not provide a complete and accurate view of allelic relationships. Attempts to reconstruct human history from genetic data must use realistic models which consider the complicating factors of nonequilibrium populations, recombination, and different patterns of selection. Received: 19 February 1997 / Accepted: 12 June 1997     

Characteristics of Nucleotide Substitution in the Hepatitis C Virus Genome: Constraints on Sequence Change in Coding Regions at Both Ends of the Genome

Comparison of complete genome sequences for different variants of hepatitis C virus (HCV) reveals several different constraints on sequence change. Synonymous changes are suppressed in coding regions at both 5′ and 3′ ends of the genome. No evidence was found for the existence of alternative reading frames or for a lower mutation frequency in these regions. Instead, suppression may be due to constraints imposed by RNA secondary structures identified within the core and NS5b genes. Nonsynonymous substitutions are less frequent than synonymous ones except in the hypervariable region of E2 and, to a lesser extent, in E1, NS2, and NS5b. Transitions are more frequent than transversions, particularly at the third position of codons where the bias is 16:1. In addition, nucleotide substitutions may not occur symmetrically since there is a bias toward G or C at the third position of codons, while T ↔ C transitions were twice as frequent as A ↔ G transitions. These different biases do not affect the phylogenetic analysis of HCV variants but need to be taken into account in interpreting sequence change in longitudinal studies. Received: 9 September 1996 / Accepted: 20 April 1997     

Sequence Analysis of the Mitochondrial Genome of Sarcophyton glaucum: Conserved Gene Order Among Octocorals

The nucleotide sequence for an 11,715-bp segment of the mitochondrial genome of the octocoral Sarcophyton glaucum is presented, completing the analysis of the entire genome for this anthozoan member of the phylum Cnidaria. The genome contained the same 13 protein-coding and 2 ribosomal RNA genes as in other animals. However, it also included an unusual mismatch repair gene homologue reported previously and codes for only a single tRNA gene. Intermediate in length compared to two other cnidarians (17,443 and 18,911 bp), this organellar genome contained the smallest amount of noncoding DNA (428, compared to 1283 and 781 nt, respectively), making it the most compact one found for the phylum to date. The mitochondrial genes of S. glaucum exhibited an identical arrangement to that found in another octocoral, Renilla kolikeri, with five protein-coding genes in the same order as has been found in insect and vertebrate mitochondrial genomes. Although gene order appears to be highly conserved among octocorals, compared to the hexacoral, Metridium senile, few similarities were found. Like other metazoan mitochondrial genomes, the A + T composition was elevated and a general bias against codons ending in G or C was observed. However, an exception to this was the infrequent use of TGA compared to TGG to code for tryptophan. This divergent codon bias is unusual but appears to be a conserved feature among two rather distantly related anthozoans. Received: 27 January 1998 / Accepted: 25 May 1998     

Rapid Rate of Control-Region Evolution in Pacific Butterflyfishes (Chaetodontidae)

Sequence differences in the tRNA-proline (tRNA^pro) end of the mitochondrial control-region of three species of Pacific butterflyfishes accumulated 33–43 times more rapidly than did changes within the mitochondrial cytochrome b gene (cytb). Rapid evolution in this region was accompanied by strong transition/transversion bias and large variation in the probability of a DNA substitution among sites. These substitution constraints placed an absolute ceiling on the magnitude of sequence divergence that could be detected between individuals. This divergence ``ceiling' was reached rapidly and led to a decay in the relative rate of control-region/cytb b evolution. A high rate of evolution in this section of the control-region of butterflyfishes stands in marked contrast to the patterns reported in some other fish lineages. Although the mechanism underlying rate variation remains unclear, all taxa with rapid evolution in the 5′-end of the control-region showed extreme transition biases. By contrast, in taxa with slower control-region evolution, transitions accumulated at nearly the same rate as transversions. More information is needed to understand the relationship between nucleotide bias and the rate of evolution in the 5′-end of the control-region. Despite strong constraints on sequence change, phylogenetic information was preserved in the group of recently differentiated species and supported the clustering of sequences into three major mtDNA groupings. Within these groups, very similar control-region sequences were widely distributed across the Pacific Ocean and were shared between recognized species, indicating a lack of mitochondrial sequence monophyly among species. Received: 30 June 1996 / Accepted: 15 May 1997     

The Heat-Shock Gene hsp83 of Drosophila auraria: Genomic Organization, Nucleotide Sequence, and Long Antiparallel Coupled ORFs (LAC ORFs)

The genomic organization of the hsp83 gene of Drosophila auraria, a far-eastern endemic species belonging to the montium subgroup of the melanogaster species group, is presented here. Based on in situ hybridization on polytene chromosomes, cDNA and genomic clone mapping, nucleotide sequencing, and genomic Southern analysis, hsp83 is shown to be present as a single-copy gene at locus 64B on the 3L chromosome arm in D. auraria. This gene is organized into two exons separated by a 929-bp intron. The first exon represents the mRNA leader sequence and is not translated, while the coding region, having a length of 2,151 bp, is solely included in the second exon. Nucleotide sequence comparisons of D. auraria hsp83 with homologous sequences from other organisms show high conservation of the coding region (88–92% identity) in the genus Drosophila, in addition to the conserved genomic organization of two-exons–one-intron, of comparable size and arrangement. A phylogenetic tree based on the protein sequences of homologous genes from representative organisms is in accord with the accredited phylogenetic position of D. auraria. In the hsp83 gene region, a second case of long antiparallel coupled open reading frames (LAC ORFs) for this species was found. The antiparallel to the hsp83 gene ORF is 1,554 bases long, while the two ORFs overlap has a size of 1,548 bp. The anti-hsp83 ORF does not show significant homology to any known gene sequences. In addition, no similar LAC ORF structures were found in homologous gene regions of other organisms. Received: 18 April 1997 / Accepted: 1 August 1997     

Complete Mitochondrial DNA Sequence of Conger myriaster (Teleostei: Anguilliformes): Novel Gene Order for Vertebrate Mitochondrial Genomes and the Phylogenetic Implications for Anguilliform Families

The complete nucleotide sequence of the mitochondrial genome was determined for a conger eel, Conger myriaster (Elopomorpha: Anguilliformes), using a PCR-based approach that employs a long PCR technique and many fish-versatile primers. Although the genome [18,705 base pairs (bp)] contained the same set of 37 mitochondrial genes [two ribosomal RNA (rRNA), 22 transfer RNA (tRNA), and 13 protein-coding genes] as found in other vertebrates, the gene order differed from that recorded for any other vertebrates. In typical vertebrates, the ND6, tRNA^Glu, and tRNA^Pro genes are located between the ND5 gene and the control region, whereas the former three genes, in C. myriaster, have been translocated to a position between the control region and the tRNA^Phe gene that are contiguously located at the 5′ end of the 12S rRNA gene in typical vertebrates. This gene order is similar to the recently reported gene order in four lineages of birds in that the latter lack the ND6, tRNA^Glu, and tRNA^Pro genes between the ND5 gene and the control region; however, the relative position of the tRNA^Pro to the ND6–tRNA^Glu genes in C. myriaster was different from that in the four birds, which presumably resulted from different patterns of tandem duplication of gene regions followed by gene deletions in two distantly related groups of organisms. Sequencing of the ND5–cyt b region in 11 other anguilliform species, representing 11 families, plus one outgroup species, revealed that the same gene order as C. myriaster was shared by another 4 families, belonging to the suborder Congroidei. Although the novel gene orders of four lineages of birds were indicated to have multiple independent origins, phylogenetic analyses using nucleotide sequences from the mitochondrial 12S rRNA and cyt b genes suggested that the novel gene orders of the five anguilliform families had originated in a single ancestral species. Received: 13 July 2000 / Accepted: 30 November 2000     

Neutral and Nonneutral Mitochondrial Genetic Variation in Deep-Sea Clams from the Family Vesicomyidae

Nucleotide sequences at two mitochondrial genes from 57 individuals representing eight species of deep-sea clams (Vesicomyidae) were examined for variation consistent with the neutral model of molecular evolution. One gene, cytochrome oxidase subunit I (COI), deviated from the expectations of neutrality by containing an excess of intraspecific nonsynonymous polymorphism. Additionally, one species, Calyptogena kilmeri, showed a significant excess of rare polymorphism specifically at the COI locus. In contrast, a second mitochondrial gene, the large-subunit 16S ribosomal RNA gene (16S), showed little deviation from neutrality either between or within species. Together, COI and 16S show no deviation from neutral expectations by the HKA test, produce congruent phylogenetic relationships between species, and show correlated numbers of fixed differences between species and polymorphism within species. These patterns of both neutral and nonneutral evolution within the mitochondrial genome are most consistent with a model where intraspecific nonsynonymous polymorphism at COI is near neutrality. In addition to examining the forces of molecular evolution, we extend hypotheses about interspecific relationships within this family for geographical locations previously unexamined by molecular methods including habitats near the Middle Atlantic, the Aleutian Trench, and Costa Rica. Received: 10 March 1999 / Accepted: 13 September 1999     

Heterogeneity in Codon Usage in the Flatworm Schistosoma mansoni

Synonymous codon choices vary considerably among Schistosoma mansoni genes. Principal components analysis detects a single major trend among genes, which highly correlates with GC content in third codon positions and exons, but does not discriminate among putatively highly and lowly expressed genes. The effective number of codons used in each gene, and its distribution when plotted against GC₃, suggests that codon usage is shaped mainly by mutational biases. The GC content of exons, GC₃, 5′, 3′, and flanking (5′+ 3′+ introns) regions are all correlated among them, suggesting that variations in GC content may exist among different regions of the S. mansoni genome. We propose that this genome structure might be among the most important factors shaping codon usage in this species, although the action of selection on certain sequences cannot be excluded. Received: 10 March 1997 / Accepted: 27 June 1997     

The complete mitochondrial DNA (mtDNA) of the donkey and mtDNA comparisons among four closely related mammalian species-pairs

The nucleotide sequence of the complete mitochondrial genome of the donkey, Equus asinus, was determined. The length of the molecule is 16,670 bp. The length, however, is not absolute due to pronounced heteroplasmy caused by variable numbers of two types of repetitive motifs in the control region. The sequence of the repeats is (a) 5′-CACACCCA and (b) 5′-TGCGCGCA, respectively. The order of (a) and (b) can be expressed as {n[2(a)+(b)]+m(a)}. In 32 different clones analyzed the number of n and m ranged from 0 to 9 and 1 to 7. The two rRNA genes, the 13 peptide-coding genes, and the 22 tRNA genes of the donkey and the horse, Equus caballus, were compared in detail. Total nucleotide difference outside the control region was 6.9%. Nucleotide difference between peptide-coding genes ranged from 6.4% to 9.4% with a mean of 8.0%. In the inferred protein sequences of the 13 peptide-coding genes the amino acid difference was 0.2–8.8%, and the mean for the 13 concatenated amino acid sequences was 1.9%. In the 22 tRNA genes, the mean difference was 3.5%, and that in the two rRNA genes was 4.1%. The mtDNA differences between the donkey and the horse suggest that the evolutionary separation of the two species occurred ≈9 million years ago. Analyses of differences among the mtDNAs of three other species-pairs, harbor seal/grey seal, fin whale/blue whale, and Homo/common chimpanzee, showed that the relative evolutionary rate of individual peptide-coding genes varies among different species-pairs and modes of comparison. The findings show that the superimposition of sequence data of one lineage for resolving and dating evolutionary divergences of other lineages should be performed with caution unless based on comprehensive data. Received: 15 October 1995 / Accepted: 15 April 1996     

A Complex Organization of the Gene Encoding Cytochrome Oxidase Subunit 1 in the Mitochondrial Genome of the Dinoflagellate, Crypthecodinium cohnii: Homologous Recombination Generates Two Different cox1 Open Reading Frames

In the course of investigating mitochondrial genome organization in Crypthecodinium cohnii, a non-photosynthetic dinoflagellate, we identified four EcoRI fragments that hybridize to a probe specific for cox1, the gene that encodes subunit 1 of cytochrome oxidase. Cloning and sequence characterization of the four fragments (5.7, 5.1, 4.1, 3.5 kilobase pairs) revealed that cox1 exists in four distinct but related contexts in C. cohnii mtDNA, with a central repeat unit flanked by one of two possible upstream (flanking domain 1 or 2) and downstream (flanking domain 3 or 4) regions. The majority of the cox1 gene is located within the central repeat; however, the C-terminal portion of the open reading frame extends into flanking domains 3 and 4, thereby creating two distinct cox1 coding sequences. The 3′-terminal region of one of the cox1 reading frames can assume an elaborate secondary structure, which potentially could act to stabilize the mature mRNA against nucleolytic degradation. In addition, a high density of small inverted repeats (15–22 base pairs) has been identified at the 5′-end of cox1, further suggesting that hairpin structures could be important for gene regulation. The organization of cox1 in C. cohnii mtDNA appears to reflect homologous recombination events within the central repeat between different cox1 sequence contexts. Such recombining repeats are a characteristic feature of plant (angiosperm) mtDNA, but they have not previously been described in the mitochondrial genomes of protists. Received: 21 December 2000 / Accepted: 30 January 2001     

Evolution of the nad3-rps12 Gene Cluster in Angiosperm Mitochondria: Comparison of Edited and Unedited Sequences

We have analyzed the nad3-rps12 locus for eight angiosperms in order to compare the utility of mitochondrial DNA and edited mRNA sequences in phylogenetic reconstruction. The two coding regions, containing from 25 to 35 editing sites in the various plants, have been concatenated in order to increase the significance of the analysis. Differing from the corresponding chloroplast sequences, unedited mitochondrial DNA sequences seem to evolve under a quasi-neutral substitution process which undifferentiates the nucleotide substitution rates for the three codon positions. By using complete gene sequences (all codon positions) we found that genomic sequences provide a classical angiosperm phylogenetic tree with a clear-cut grouping of monocotyledons and dicotyledons with Magnoliidae at the basal branch of the tree. Conversely, owing to their low nucleotide substitution rates, edited mRNA sequences were found not to be suitable for studying phylogenetic relationships among angiosperms. Received: 24 January 1996 / Accepted: 5 June 1996     

Plant Mitochondrial RNA Editing

RNA editing affects messenger RNAs and transfer RNAs in plant mitochondria by site-specific exchange of cytidine and uridine bases in both seed and nonseed plants. Distribution of the phenomenon among bryophytes has been unclear since RNA editing has been detected in some but not all liverworts and mosses. A more detailed understanding of RNA editing in plants required extended data sets for taxa and sequences investigated. Toward this aim an internal region of the mitochondrial nad5 gene (1104 nt) was analyzed in a large collection of bryophytes and green algae (Charales). The genomic nad5 sequences predict editing in 30 mosses, 2 hornworts, and 7 simple thalloid and leafy liverworts (Jungermanniidae). No editing is, however, required in seven species of the complex thalloid liverworts (Marchantiidae) and the algae. RNA editing among the Jungermanniidae, on the other hand, reaches frequencies of up to 6% of codons being modified. Predictability of RNA editing from the genomic sequences was confirmed by cDNA analysis in the mosses Schistostega pennata and Rhodobryum roseum, the hornworts Anthoceros husnotii and A. punctatus, and the liverworts Metzgeria conjugata and Moerckia flotoviana. All C-to-U nucleotide exchanges predicted to reestablish conserved codons were confirmed. Editing in the hornworts includes the removal of genomic stop codons by frequent reverse U-to-C edits. Expectedly, no RNA editing events were identified by cDNA analysis in the marchantiid liverworts Ricciocarpos natans, Corsinia coriandra, and Lunularia cruciata. The findings are discussed in relation to models on the phylogeny of land plants. Received: 2 April 1998 / Accepted: 4 August 1998     

Rates of Nucleotide Substitution in Angiosperm Mitochondrial DNA Sequences and Dates of Divergence Between Brassica and Other Angiosperm Lineages

We obtained 16 nucleotide sequences (∼1400 bp each) of the first intron of the mitochondrial (mt) gene for NADH subunit 4 (nad4) from 10 species of Brassicaceae. Using these new sequences and five published sequences from GenBank, we constructed a phylogenetic tree of the Brassicaceae species under study and showed that the rate of nucleotide substitution in the first intron of nad4 is very low, about 0.16–0.23 × 10⁻⁹ substitution per site per year, which is about half of the silent rate in exons of nad4. The ratios of substitution rates in this intron, ITS, and IGS are approximately 1:23:73, where ITS is the nuclear intergenic spacer between 18S and 25S rRNA genes and IGS is the intergenic spacer of 5S rRNA genes. A segment (335 bp) in the first intron of nad4 in Brassicaceae species that is absent in wheat was considered as a nonfunctional sequence and used to estimate the neutral rate (the rate of mutation) in mtDNA to be 0.5–0.7 × 10⁻⁹ substitution per site per year, which is about three times higher than the substitution rate in the rest of the first intron of nad4. We estimated that the dates of divergence are 170–235 million years (Myr) for the monocot–dicot split, 112–156 Myr for the Brassicaceae–Lettuce split, 14.5–20.4 Myr for the Brassica–Arabidopsis split, and 14.5–20.4 Myr for the Arabidopsis–Arabideae split. Received: 14 July 1998 / Accepted: 1 October 1998     

Phylogenomic Analysis of the α Proteasome Gene Family from Early-Diverging Eukaryotes

We employed a phylogenomic approach to study the evolution of α subunits of the proteasome gene family from early diverging eukaryotes. BLAST similarity searches of the Giardia lamblia genome identified all seven α proteasome genes characteristic of eukaryotes from the crown group. In addition, a PCR strategy for the amplification of multiple α subunit sequences generated single α proteasome products for representatives of the Kinetoplastida (Leishmania major), the Parabasalia (Trichomonas vaginalis), and the Microsporidia (Vairimorpha sp., Nosema sp., Endoreticulata sp., and Spraguea lophii). The kinetoplastid Trypanosoma cruzi and the eukaryote crown group Acanthamoeba castellanii yielded two distinct α proteasome genes each. The presence of seven distinct α proteasome genes in G. lamblia, one of the earliest-diverging eukaryotes, indicates that the α proteasome gene family evolved rapidly from a minimum of one gene in Archaea to seven or more in Eukarya. Results from the phylogenomic analysis are consistent with the idea that the Diplomonida (as represented by G. lamblia), the Kinetoplastida, the Parabasalia, and the Microsporidia diverged after the duplication events that originated the α proteasome gene family. A model for the early origin and evolution of the proteasome gene family is presented. Received: 14 February 2000 / Accepted: 14 August 2000     

Molecular Evolution of the Aldo-keto Reductase Gene Superfamily

The aldo-keto reductase enzymes comprise a functionally diverse gene family which catalyze the NADPH-dependant reduction of a variety of carbonyl compounds. The protein sequences of 45 members of this family were aligned and phylogenetic trees were deduced from this alignment using the neighbor-joining and Fitch algorithms. The branching order of these trees indicates that the vertebrate enzymes cluster in three groups, which have a monophyletic origin distinct from the bacterial, plant, and invertebrate enzymes. A high level of conservation was observed between the vertebrate hydroxysteroid dehydrogenase enzymes, prostaglandin F synthase, and ρ-crystallin of Xenopus laevis. We infer from the phylogenetic analysis that prostaglandin F synthase may represent a recent recruit to the eicosanoid biosynthetic pathway from the hydroxysteroid dehydrogenase pathway and furthermore that, in the context of gene recruitment, Xenopus laevisρ-crystallin may represent a shared gene. Received: 26 August 1996 / Accepted: 5 June 1997