New developments in vertebrate cytotaxonomy VII

The chromosome formulae of 721 species and subspecies are reported, bringing the number of cytologically known rodent species and subspecies to 1 341, the inventory of Matthey (Mammalia 40: 453–466, 1976) taken into account. In 67.40% of the taxa listed chromosomal analysis has been performed according to the classical mode whereas in 32.60% banding methods have been used. The great increase in data on rodent chromosomes observed in the last ten years is attributed to three factors: the advent of differential staining, the utility of the karyotype in the taxonomic definition of new species and subspecies, and the existence of chromosome polymorphism. The results presented show that the contribution of banding methods to cytotaxonomic analysis is considerable. Henceforth the problem of interspecific phyletic relations will be stated in terms of genomic organization, DNA diversity, chromatin nature and repartition of different genome fractions, the relative value of the numerical criteria (2n and FN) having consequently diminished.

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This is the seventh in a series of articles, by different authors, to appear in this journal under the editorship of Professor Chiarelli, supplementing the data published in Cytotaxonomy and Vertebrate Evolution (A. B. Chiarelli & E. Capanna, eds, Academic Press, London & New York, 1973) Eds.     

New developments in vertebrate cytotaxonomy VIII. A current list of references on avian karyology

A list of 256 references dealing with avian karyology is presented. This list is updated to April 1983 and covers 587 bird species, 385 non-passerines and 202 passerines. It includes 37 non-passerines and 29 passerines that were only studied in the period from 1902 onwards to the early 1950s using less sophisticated techniques (mostly on sectioned material of gonadal tissues). The remaining 348 non-passerines and 173 passerines were studied with more advanced techniques using colchicine-treated material of feather-pulp, embryonic tissue, bone-marrow, leucocyte or tissue culture. The species studied to date belong to 25 of the 26 avian orders and to 96 of the 160 avian families.This is the eighth in a series of articles, by different authors, to appear in this journal under the editorship of Professor Chiarelli, supplementing the data published in Cytotaxonomy and Vertebrate Evolution (A. B. Chiarelli & E. Capanna, eds. Academic Press, London & New York, 1973)-Eds.     

New developments in vertebrate cytotaxonomy IX Chromosome numbers in the order Insectivora (Mammalia)

A reference list of chromosome numbers of about 110 species in the order insectivora is presented. Cytotaxonomic data are known for six families: Solenodontidae, Tenrecidae (incl. Potamogalidae), Erinaceidae, Soricidae, Talpidae and Macroscelididae (only the Chrysochloridae remain unstudied). The list provides, wherever possible, the diploid chromosome number(s), Nombre Fondamental, and references; sometimes information on geography, synonymy, as well as short comments are added.This is the ninth in a series of articles, by different authors, to appear in this journal under the editorship of Professor Chiarelli, supplementing the data published in Cytotaxonomy and Vertebrate Evolution (A. B. Chiarelli & E. Capanna, eds, Academic Press, London & New York, 1973).-Eds.     

The chromosomes of some lower chordates

The chromosomes of three species of lower chordates were examined using a squash technique on small pieces of testis. Ciona intestinalis, a tunicate of the order Enterogona, has fourteen pairs of minute chromosomes. Styela plicata, a tunicate of the order Pleurogona, has sixteen pairs of chromosomes whose total size is approximately twice that of the Ciona chromosomes and about 10% of that of a typical mammalian complement. The hagfish, Eptatretus stoutii, of the suborder Myxinoidea, order Cyclostomata, has twenty-four pairs of chromosomes and what appear to be one to four small supernumeraries in some animals. The hagfish chromosomes are large, approaching the size of a typical mammalian complement. These size relationships agree in general with a concept of a small ancestral vertebrate genome which evolved into the larger present day genomes through a series of duplications of genetic material.This work was supported in part by Grant No. 247-701 from the San Diego State College Foundation.     

Cytotaxonomy and evolution in Cryptocoryne (Araceae)

A hypothetical phylogeny is presented for the genus Cryptocoryne (Araceae, Cryptocoryninae). This scheme is based on both geographical and cytological data. Therefore the geography of S. E. Asia, the distribution of the various base numbers and the possible relationships between the base numbers are discussed.In the resulting phylogeny the various base numbers of Cryptocoryne (and the one of Lagenandra) are thought to have been derived from a hypothetical primary base number x₁=9. As a consequence of the scheme, Cryptocoryne is assumed to have a diphyletic character.     

The terminal neuromuscular junctions of lower chordates

Summary The structure of the terminal neuromuscular junctions found close to the insertions of myotomal muscle fibres in teleost fish and amphibia is described, and it is concluded that in both groups, the pattern of innervation is essentially similar. Different degrees of complexity are described, both in the terminal nerve branches, and in the sub-junctional apparatus. It is suggested that the terminal pattern of innervation is an adaptation to the requirements of rapid swimming.     

Retinoic acid signaling and the evolution of chordates

In chordates, which comprise urochordates, cephalochordates and vertebrates, the vitamin A-derived morphogen retinoic acid (RA) has a pivotal role during development. Altering levels of endogenous RA signaling during early embryology leads to severe malformations, mainly due to incorrect positional codes specifying the embryonic anteroposterior body axis. In this review, we present our current understanding of the RA signaling pathway and its roles during chordate development. In particular, we focus on the conserved roles of RA and its downstream mediators, the Hox genes, in conveying positional patterning information to different embryonic tissues, such as the endoderm and the central nervous system. We find that some of the control mechanisms governing RA-mediated patterning are well conserved between vertebrates and invertebrate chordates, such as the cephalochordate amphioxus. In contrast, outside the chordates, evidence for roles of RA signaling is scarce and the evolutionary origin of the RA pathway itself thus remains elusive. In sum, to fully understand the evolutionary history of the RA pathway, future research should focus on identification and study of components of the RA signaling cascade in non-chordate deuterostomes (such as hemichordates and echinoderms) and other invertebrates, such as insects, mollusks and cnidarians.     

Homeobox genes in vertebrate evolution.

A wide range of anatomical features are shared by all vertebrates, but absent in our closest invertebrate relatives. The origin of vertebrate embryogenesis must have involved the evolution of new regulatory pathways to control the development of new features, but how did this occur? Mutations affecting regulatory genes, including those containing homeobox sequences, may have been important: for example, perhaps gene duplications allowed recruitment of genes to new roles. Here I ask whether comparative data on the genomic organization and expression patterns of homeobox genes support this hypothesis. I propose a model in which duplications of particular homeobox genes, followed by the acquisition of gene-specific secondary expression domains, allowed the evolution of the neural crest, extensive organogenesis and craniofacial morphogenesis. Specific details of the model are amenable to testing by extension of this comparative approach to molecular embryology.     

Cytotaxonomy and karyotype evolution inPhleum sect.Phleum (Poaceae) in Poland

Three taxa are distinguished in the sectionPhleum Griseb. in Poland:P. nodosum (2n = 14),P. pratense (2n = 42), and the third one unformally named hereP. commutatum (2n = 14). It corresponds morphologically toP. commutatum Gaud. reported as a tetraploid taxon (2n = 28) from other geographic regions. Giemsa C-banded karyotypes of these three taxa help clarify the taxonomic status ofP. commutatum and the origin of the hexaploidP. pratense. It is suggested that changes in the amount of telomeric heterochromatin played an important role in the evolution ofPhleum karyotypes.     

Modularity in vertebrate brain development and evolution.

Embryonic modularity and functional modularity are two principles of brain organization. Embryonic modules are histogenetic fields that are specified by position-dependent expression of patterning genes. Within each embryonic module, secondary and higher-level pattern formation takes places during development, finally giving rise to brain nuclei and cortical layers. Defined subsets of these structures become connected by fiber tracts to form the information-processing neural circuits, which represent the functional modules of the brain. We review evidence that a group of cell adhesion molecules, the cadherins, provides an adhesive code for both types of modularity, based on a preferentially homotypic binding mechanism. Embryonic modularity is transformed into functional modularity, in part by translating early-generated positional information into an array of adhesive cues, which regulate the binding of functional neural structures distributed across the embryonic modules. Brain modularity may provide a basis for adaptability in evolution.