The developmental control genes containing an Antennapedia-type homeobox are clustered in insects andvertebrates. The evolution of these genes was studied by the construction ofevolutionary trees and by statistical geometry in sequence space. The comparative analysis of the homeobox sequences reveals the subdivision of the Antennapedia-type homeobox genes into three classes early in metazoan evolution. This observation suggests an important function of these genes even in the most primitive metazoans. Subsequent duplication events generated a cluster of at least five homeobox genes in the last common ancestor of insects and vertebrates. These genes later independently gave rise to the 13 groups of paralogous genes in vertebrates and to the 11 Antennapedia-type genes in the Drosophila complexes.The homeobox consists ofa 183-base-pair sequence encoding a trihelical DNA binding motif. This domain has been found in several genes in a wide variety ofeukaryotic organisms (for reviews see refs. 1 and 2). The protein products of some of the homeobox genes act as ubiquitous transcription factors, whereas most are involved in the control of embryonic development. In the fruit fly Drosophila as well as in the house mouse, the segment identity along the anteriorposterior axis of the embryo is specified by homeotic selector genes belonging to the Antennapedia-type of homeobox genes (reviewed in ref.3). Both in insects (HOM genes) and in vertebrates (Hox genes) these genes are located in clusters. The beetle Tribolium (4, 5) has its homeotic genes arranged in one complex (HOM-C), whereas Drosophila has them split into two complexes [Bithorax complex (BX-C) and Antennapedia complex (ANT-C)]. In mammals, the four clusters (Hox-1 to Hox-4) can be aligned according to sequence similarities of their genes, thus defining 13 groups of paralogous genes. Kappen et al. (6) have therefore suggested that cluster duplication events underlie the evolution of the vertebrate Hox genes. A closer examination of the homeobox gene clusters in insects and vertebrates revealed remarkable resemblances (7,8). The Drosophila HOM genes are arranged in a similar order on the chromosome as their vertebrate counterparts. Moreover, in both organisms, these genes exhibit a correlation between their expression boundaries and their position in the cluster: generally the further downstream (3') a gene is in its cluster, the more anterior its expression boundary is in the embryo (for reviews see refs. 9 and 10). A common ancestral cluster for insects and vertebrates therefore appears to be highly probable (7,8,11). Reports of a related homeobox gene cluster in the nematode Caenorhabditis (12) and ofAntennapedia-type homeobox genes in cnidarians (13, 14) support this hypothesis.To gain insight into the evolution of developmental control mechanisms, as well as the phylogenetic relationships between the metazoans, several studies of the evolution of the Antennapedia-type homeobox genes were done by comparative sequence analyses (1,6,11,(15)(16)(17). While...

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HIV-2ALT is a highly divergent HIV-2-related isolate that is genetically equidistant to the prototypic HIV-2 strains, defined by HIV-2ROD, and to the simian immunodeficiency viruses SIVmac and SIVsm. We have now cloned and sequenced the envelope region of HIV-2ALT, thus completing the analysis of the whole viral genome. The sequences of env and nef and of the second exons of tat and rev were compared with those of the other viruses of the HIV-2/SIVsm/SIVmac group. Despite of the high degree of variation of HIV-2ALT, functional domains of the genes are conserved. Although in env, the overall pattern of constant and variable domains is maintained, many single amino acid exchanges exist at positions previously thought to be constant in HIV-2 strains. In addition, when compared with a broader spectrum of immunodeficiency viruses, which includes SIVMND from mandrill and SIVAGM from African green monkey, HIV-2ALT Env has a high percentage of amino acid exchanges, which are unique to this strain. This underlines the separate branch of HIV-2ALT within the phylogenetic tree and makes obvious the inclusion of such divergent strains in preventive and therapeutic programs.

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