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Changes in Hox genes’ structure and function during the evolution of the squamate body plan

Nature, 2010

Hox genes are central to the specification of structures along the anterior-posterior body axis 1,2 , and modifications in their expression have paralleled the emergence of diversity in vertebrate body plans 3,4 . Here we describe the genomic organization of Hox clusters in different reptiles and show that squamates have accumulated unusually large numbers of transposable elements at these loci 5 , reflecting extensive genomic rearrangements of coding and non-coding regulatory regions. Comparative expression analyses between two species showing different axial skeletons, the corn snake and the whiptail lizard, revealed major alterations in Hox13 and Hox10 expression features during snake somitogenesis, in line with the expansion of both caudal and thoracic regions. Variations in both protein sequences and regulatory modalities of posterior Hox genes suggest how this genetic system has dealt with its intrinsic collinear constraint to accompany the substantial morphological radiation observed in this group.

Hox genes and the evolution of vertebrate axial morphology

Development (Cambridge, England), 1995

A common form of evolutionary variation between vertebrate taxa is the different numbers of segments that contribute to various regions of the anterior-posterior axis; cervical vertebrae, thoracic vertebrae, etc. The term 'transposition' is used to describe this phenomenon. Genetic experiments with homeotic genes in mice have demonstrated that Hox genes are in part responsible for the specification of segmental identity along the anterior-posterior axis, and it has been proposed that an axial Hox code determines the morphology of individual vertebrae (Kessel, M. and Gruss, P. (1990) Science 249, 347-379). This paper presents a comparative study of the developmental patterns of homeobox gene expression and developmental morphology between animals that have homologous regulatory genes but different morphologies. The axial expression boundaries of 23 Hox genes were examined in the paraxial mesoderm of chick, and 16 in mouse embryos by in situ hybridization and immunolocalizatio...

Shaping animal body plans in development and evolution by modulation of Hox expression patterns

Bioessays, 1998

Most animals exhibit distinctive and diverse morphological features on their anterior-posterior body axis. However, underneath the variation in design and developmental strategies lies a shared ancient structural blueprint that is based on the expression patterns of Hox genes. Both the establishment and maintenance of the spatial and temporal distribution of Hox transcripts play an important role in determining axial pattern. The study of many animal systems, both vertebrate and invertebrate, suggests that the mechanisms used to establish Hox transcription are nearly as diverse as the body plans they specify. The strategies for maintenance of Hox expression pattern seem more conserved among different phyla, and rely on the action of Pc and trx group genes as well as auto-and cross-regulation among Hox genes. In mice, the sharing of regulatory elements coupled with auto-and cross-regulation could explain the conservation of the clustered arrangement of Hox genes. In contrast, fly Hox genes seem to have evolved insulators or boundary elements to avoid sharing regulatory regions. Differences in Hox transcription patterns can be correlated with morphological modifications in different species, and it seems likely that evolutionary variation of Hox cis-regulatory elements has played a major role in the emergence of novel body plans in different taxa of the animal kingdom.

Comparison of Models for the Collinearity of Hox Genes in the Developmental Axes of Vertebrates

Current Genomics, 2012

Hox gene clusters are very frequent in many animal genomes and their role in development is pivotal. Particularly in vertebrates, intensive efforts have established several properties of Hox clusters. The collinearity of Hox gene expressions (spatial, temporal and quantitative) is a common feature of the vertebrates. During the last decade, genetic engineering experiments have revealed some important facets of collinearity during limb and trunk development in mice. Two models have been proposed to explain all these properties. On one hand the 'two-phases model' makes use of the molecular regulatory mechanisms acting on the Hox genes. On the other hand, the'biophysical model' is based on the signals transduced inside the cell nucleus and the generation of forces which apply on the cluster and lead to a coordinated activation of Hox genes. The two models differ fundamentally and a critical and detailed comparison is presented. Furthermore, experiments are proposed for which the two models provide divergent predictions. The outcome of these experiments will help to decide which of the two models is valid (if any).

On the Homology of Structures and Hox Genes: The Vertebral Column

Novartis Foundation Symposia, 2007

Research on expression patterns of Hox genes has revealed a surprisingly high conservation among vertebrates. In agreement with this conservation, a correlation has been found between the anterior limits of expression areas of certain Hox genes and the borders between morphological regions of the vertebral axis. These similarities are striking and important, but also counterintuitive, unless there are strong selection pressures to protect this conservatism. It is important to identify the selective forces that maintain these conservative networks. These selective forces can be due to pleiotropy or to internal selection. Discussed are the selective factors that are involved in the evolutionary constraint on the number of cervical vertebral numbers in mammals. Factors involved are due to internal selection and involve susceptibility to cancer, stillbirths and neuronal problems. It is intriguing how similar genetic networks can lead to fundamentally different animals. Clearly the same genes are used for different purposes. It is therefore important to try to find these differences. The search for homology between organisms, and the enthusiasm about similarities that come with it, at times impedes the discovery of such differences. I have searched the literature for differences within vertebrates in the functioning and expression patterns of Hox genes during the development of the vertebral axis. The ensuing implications for homology of structures and genes are discussed. The vertebral column is a promising model system for the evaluation of the relationship between homologous Hox genes and homologous structures because of the large conservation of Hox gene expression patterns along the anterior axis. However, extensive remodelling of the vertebrate column indicates that important changes in the genetic basis must have taken place. A survey of the literature indicates that the correlation between Hox gene expression areas and vertebral regions is not such that one can predict the borders between vertebral regions on the basis of Hox gene expression patterns. The involvement of Hox genes in the development of identity of vertebrae is complex and the problems regarding the value of gene expression patterns for the determination of anatomical homology are discussed.

Teleost HoxD and HoxA genes: comparison with tetrapods and functional evolution of the HOXD complex

Mechanisms of Development, 1996

In tetrapods, Hux genes are essential for the proper organization and development of axial structures. Experiments involving Hox gene inactivations have revealed their particularly important functions in the establishment of morphological transitions within metameric series such as the vertebral column. Teleost fish show a much simpler range of axial (trunk or appendicular) morphologies, which prompted us to investigate the nature of the Hex system in these lower vertebrates. Here, we show that iish have a family of Hox genes, very similar in both number and general organization, to that of tetrapods. Expression studies, carried out with HoxD and HOXA genes, showed that all vertebrates use the same general scheme, involving the colinear activation of gene expression in both space and time. Comparisons between tetrapods and fish allowed us to propose a model which accounts for the primary function of this gene family. In this model, a few ancestral Hox genes were involved in the determination of polarity in the digestive tract and were further recruited in more elaborate axial structures.