Medaka and zebrafish, an evolutionary twin study

Comparison of two related species is one of the most successful approaches to decipher general genetic principles in eukaryotes. This is best illustrated in yeast, where the model systems Saccharomyyces. cervisiae and Schizosaccharomyces. pombe have been examined. Powerful forward genetics in both species, species-specific differences in biological features and the phylogenetic distance between the two species, make them well suited for a comparative approach. Recent whole genome sequencing has also facilitated comparative genomics of these simple eukaryotes. It is now possible to go a step further using higher eukaryotes. A duplication of the genome at the base of the teleost radiation, facilitated evolution of almost 25,000 fish species, more than half of all vertebrate species together. Two teleost genetic model systems have emerged in the past few decades: zebrafish, in which large-scale mutagenesis has been successfully performed, and Medaka, a Japanese killifish with a century of history in genetics and now, as reported in this issue, many induced mutations. In this review we will illustrate how comparison of these two model species, Medaka and zebrafish, can reveal conserved and species-specific genetic and molecular mechanisms underlying vertebrate development.     

Seasonal variation of livebirths, stillbirths, extramarital births and twin maternities in Switzerland.

A study was made of the seasonal variation in all births, and births according to marital status, multiplicity and birth status (live and still) in Switzerland recorded between 1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses are based on rates. When comparing the seasonality in data sets showing markedly different levels, standardised indices were used. Assuming the length of pregnancies with twins to be about one month shorter than for pregnancies with singletons, lagged twinning rates were calculated but, in comparison with actual twinning rates, the general seasonal variation remained. Therefore, this study was based on actual twinning rates. A monotonic increase in the amplitude of the seasonal variation in general births was noted for the period 1876-1930, with strong seasonal variation holding for 1921-1980. After that, a marked decline in the amplitude can be observed. Seasonality of both all births and twin maternities showed very similar pattern for the periods 1876-1930 and 1969-1990, with maxima in the spring (March-May) and troughs in late autumn (October-December). Twin maternities showed a strong seasonality for the period 1876-1930, being about 20% higher in March than in October. The twinning rate in the period 1876-1930 was about 2.6 per thousand units higher than in the period 1969-90. For twin maternities there was also a stronger seasonal variation during the earlier period than during the later one. The pattern of the seasonal variation for extramarital births, showing a maximum in February (conceptions in May-June) and a minimum in August (conceptions in November-December) with a difference of no less than 24% was more marked than for the marital births. It seems likely that this seasonality of extra-marital maternities was due mainly to seasonal variation of coital rates and multiple ovulation in the early summer months coinciding with optima of light, temperature and food supply. A strong reduction in the rate of stillbirths (gestational age more than 29 weeks) was observed during the twentieth century. The stillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 in the 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonal rhythm was noticed throughout the period with high stillbirth rates among births around March and low rates during the summer and autumn.     

Russian twin studies: colleagues, controversies, case studies and current events.

The 2001 honoring of Russian twin researcher, Dr. Inna V. Ravich-Shcherbo, at the Tenth International Congress on Twin Studies (ISTS), in London, brought timely recognition to an international colleague. It also marked an occasion for reflecting on the course of twin studies in Russia. Historical trends and current accomplishments are examined with an eye toward future developments. Next, the distinguished careers of Russian monozygotic twins, Drs. Alexander and Andrew Fingelkurts, exemplify twin research findings on intelligence and occupational choice, and illuminate the status of twin studies and other scientific research in their country, are told. Their life histories are followed by the initially tragic, but ultimately heartwarming, story of young twins, Max and Andy, whose physical disabilities were overcome through the efforts of an empathic war veteran. The recent Moscow Summer School, the first in a three-time lecture series, encouraged crucial academic exchange among scientific investigators and students from around the world. Final thoughts are that much can be learned and much can be accomplished, given that we continue to nurture the twin-related resources available to us.     

Analysis of twin data using SAS

Summary .  Twin studies are essential for assessing disease inheritance. Data generated from twin studies are traditionally analyzed using specialized computational programs. For many researchers, especially those who are new to twin studies, understanding and using those specialized computational programs can be a daunting task. Given that SAS (Statistical Analysis Software) is the most popular software for statistical analysis, we suggest that the use of SAS procedures for twin data may be a helpful alternative and demonstrate that we can obtain similar results from SAS to those produced by specialized computational programs. This numerical validation is practically useful, because a natural concern with general statistical software is whether it can deal with data that are generated from special study designs such as twin studies and if it can test a particular hypothesis. We concluded through our extensive simulation that SAS procedures can be used easily as a very convenient alternative to specialized programs for twin data analysis.     

Classical twin studies and beyond

Twin studies have been a valuable source of information about the genetic basis of complex traits. To maximize the potential of twin studies, large, worldwide registers of data on twins and their relatives have been established. Here, we provide an overview of the current resources for twin research. These can be used to obtain insights into the genetic epidemiology of complex traits and diseases, to study the interaction of genotype with sex, age and lifestyle factors, and to study the causes of co-morbidity between traits and diseases. Because of their design, these registers offer unique opportunities for selected sampling for quantitative trait loci linkage and association studies.     

A twin study of dental dimension. I. Discordance,asymmetry, and mirror imagery

Bilateral tooth measurements in twins were partitioned into three orthogonal contrasts, each associated with one degree of freedom, to estimate three parameters: discordance, asymmetry, and mirror imagery. The probability levels of the within-pair variance ratios were used to test for significance of these estimates. The results provided strong evidences for the existence of significant genetic determinants of almost all of the individual tooth dimensions, but little or no evidence for a genetic basis of asymmetry. The analysis gave no indication that monozygotic twinning was associated with an increased degree of either fluctuating asymmetry or mirror imagery, when compared to dizygotic twins. The data on monozygotic twins further suggested that for most variables examined, the increment of environmental discordance resulting from the twinning phenomena was greater than the developmental noise that caused asymmetry within individual cotwins.