A molecular approach to the evolution of vertebrate paired appendages

Over the past few years, genes involved in the ontogenesis of tetrapod limbs have been Isolated and characterized. Some of the developmental mechanisms responsible for the morphogenesis of these complex structures can now be investigated through a new approach. In addition, these genes can serve as tools to re-evaluate some aspects of the long-standing question of the fin-to-limb transition. Comparative molecular developmental biology is providing new insight into the similarities and differences in the morphologies of these homologous structures.     

Developmental genetics and homology: a hierarchical approach

New advances in developmental genetics are providing a bridge to connect the study of development and evolution. The successful integration of these fields, however, is dependent on having a clear understanding of the concept of homology. Therefore, developmental genetic data must be placed within the context of the comparative method to provide insight into the evolutionary and developmental origins of traits. The comparative analysis of traits derived from several hierarchical levels (genes, gene expression patterns, embryonic origins and morphology) can potentially reveal scenarios of developmental integration, opportunity and constraint. Moreover, this approach has implications for resolving modern controversies surrounding the concept of homology.     

The evolution of the vertebrates--genes and development

In the past year, studies on protochordates have provided evidence that many features that we take to be indicative of the vertebrates were evident early in chordate evolution. Furthermore, many of the important developmental regulatory genes have also been identified in these invertebrates. Finally, we are also gaining a better insight into how the vertebrate genome itself evolved.     

Genetic insights into the functional elements of language

Language disorders cover a wide range of conditions with heterologous and overlapping phenotypes and complex etiologies harboring both genetic and environmental influences. Genetic approaches including the identification of genes linked to speech and language phenotypes and the characterization of normal and aberrant functions of these genes have, in recent years, unraveled complex details of molecular and cognitive mechanisms and provided valuable insight into the biological foundations of language. Consistent with this approach, we have reviewed the functional aspects of allelic variants of genes which are currently known to be either causally associated with disorders of speech and language or impact upon the spectrum of normal language ability. We have also reviewed candidate genes associated with heritable speech and language disorders. In addition, we have evaluated language phenotypes and associated genetic components in developmental syndromes that, together with a spectrum of altered language abilities, manifest various phenotypes and offer details of multifactorial determinants of language function. Data from this review have revealed a predominance of regulatory networks involved in the control of differentiation and functioning of neurons, neuronal tracks and connections among brain structures associated with both cognitive and language faculties. Our findings, furthermore, have highlighted several multifactorial determinants in overlapping speech and language phenotypes. Collectively this analysis has revealed an interconnected developmental network and a close association of the language faculty with cognitive functions, a finding that has the potential to provide insight into linguistic hypotheses defining in particular, the contribution of genetic elements to and the modular nature of the language faculty.     

Developmental genetics and early hominid craniodental evolution

Although features of the dentition figure prominently in discussions of early hominid phylogeny, remarkably little is known of the developmental basis of the variations in occlusal morphology and dental proportions that are observed among taxa. Recent experiments on tooth development in mice have identified some of the genes involved in dental patterning and the control of tooth specification. These findings provide valuable new insight into dental evolution and underscore the strong developmental links that exist among the teeth and the jaws and cranium. The latter has important implications for cladistic studies that traditionally consider features of the skull independently from the dentition.     

Tissue development and RNA control: "HOW" is it coordinated?

The regulation of developmental processes at the RNA level enables selective and rapid modulation of gene expression. Studies in model organisms revealed the essential contribution of the signal transduction and activation of RNA (STAR) family of RNA binding proteins to developmental processes. STAR proteins coordinate the proper timing of developmental events by delaying expression or altering the mRNA or protein levels of essential genes. Recent functional analysis of the Drosophila melanogaster STAR protein, Held Out Wing (HOW), in the context of embryonic development, provided insight into its mode of activity. Here, we describe HOW's activity in the temporal repression or elevation of gene expression that is essential for coordinating the correct timing of instructive signals.     

文昌鱼GFP基因的鉴定及表达分析

近年在隶属头索动物亚门的文昌鱼体内发现有内源性绿色荧光蛋白存在,并发现文昌鱼荧光蛋白的发光现象在不同发育时期以及个体间有较大的差异。为了进一步揭示GFP基因在文昌鱼中的进化模式,探索其可能执行的功能,该文首先对白氏文昌鱼(Branchiostoma belcheri)GFP基因作了全面鉴定,并对其不同发育阶段胚胎及成体不同区域中的荧光信号进行了实时观察记录,进而对GFP基因在绿色荧光表达强烈的两个特定时期做了绝对定量检测。研究结果表明,文昌鱼基因组中至少有12个内源性GFP基因,在个体发育的不同时期,内源性荧光出现的位置有所变化,而且在变态后的个体之间出现荧光的情况差异较大,荧光蛋白基因的表达由多个GFP同源基因共同参与,这些基因在不同的发育时期表达量有较大的差异,提示不同的GFP基因在特定发育阶段可能行使各自的功能。     

cDNA fingerprinting of osteoprogenitor cells to isolate differentiation stage-specific genes.

A cDNA fingerprinting strategy was developed to identify genes based on their differential expression pattern during osteoblast development. Preliminary biological and molecular staging of cDNA pools prepared by global amplification PCR allowed discrim-inating choices to be made in selection of expressed sequence tags (ESTs) to be isolated. Sequencing of selected ESTs confirmed that both known and novel genes can be isolated from any developmental stage of interest, e.g. from primitive progenitors, intermediate precursors or mature osteoblasts. EST expression provides insight into possible interrelated physiological functions and putative interacting molecules during differentiation. This method offers a functional genomics approach to isolate differentiation stage-specific genes in samples as small as a single cell.     

Evolutionary dynamics of genes controlling floral development

Advances in the understanding of floral developmental genetics in model species such as Arabidopsis continue to provide an important foundation for comparative studies in other flowering plants. In particular, floral organ identity genes are the focus of many projects that are addressing both ancient and recent evolutionary questions. Expanded analyses of the evolution of these gene lineages have highlighted the dynamic nature of the gene birth-and-death process, and may have significant implications for the evolution of genetic pathways. Crucial functional studies of floral organ identity genes in diverse taxa are allowing the first real insight into the conservation of gene function, while findings on the genetic control of organ elaboration offer to open up new avenues for investigation. Taken together, these trends show that the field of floral developmental evolution continues to make significant progress towards elucidating the processes that have shaped the evolution of flower development and morphology.     

Molecular evolution of flower development

Flowers, as reproductive structures of the most successful group of land plants, have been a central focus of study for both evolutionists and ecologists. Recent advances in unravelling the genetics of flower development have provided insight into the evolution of floral structures among angiosperms. The study of the evolution of genes that control floral morphogenesis permits us to draw inferences on the diversification of developmental systems, the origin of floral organs and the selective forces that drive evolutionary change among these plant reproductive structures.     

129/Sv mice—a model system for studying germ cell biology and testicular cancer

Some forms of testicular germ cell tumors (TGCTs) arise from primordial germ cells (PGCs) during fetal development. In both humans and mice, genetic control of susceptibility is complex, involving both Mendelian and polygenic factors. Identification and characterization of TGCT genes will provide insight not only into the basis for inherited susceptibility, but also into the genetic control of the development of the PGC lineage. Recent work has revealed the identity of several susceptibility genes that are inherited as Mendelian traits, the chromosomal location of yet-to-be identified TGCT susceptibility genes, as well as clues to the nature of developmental pathways involved in tumorigenesis. In this review we summarize current understanding of the biology and genetics of TGCTs in mice and discuss the relevance of this work to testicular cancer in humans. Received: 18 September 2000 / Accepted: 4 October 2000