COLONIZATION OF AMERICA BY DROSOPHILA SUBOBSCURA: LETHAL GENES AND THE PROBLEM OF THE O5 INVERSION

In this work, the process of colonization of North and South America by the species Drosophila subobscura has been studied by analyzing the variability of lethal genes. The genetic structures of a Palearctic natural central population (Bordils, Spain) and a colonizer population from America (Gilroy, California) have been compared. The frequencies of lethal chromosomes and their allelism are 29.007% and 0.0069 in the first population and 14.414% and 0.0526 in the American population. A founder effect is detected after the computation of some population parameters (N_e , h, he and the lethal load). Furthermore, the allelism of lethal chromosomes has revealed a strong association between a lethal gene and the O₅ inversion both in Gilroy and in the population of Puerto Montt (Chile). The interpopulation allelism shows that the O₅ arrangement from the USA and Chile is the same, confirming that the colonizing processes of North and South America are correlated. The O₅ arrangement can also be useful as a genetic marker to trace the origin of the colonization. The frequency of the O₅ arrangement in the original population of the colonization could be used to estimate the number of colonizers. This population is still unknown, but taking the extreme values of the frequency of the O₅ inversion in natural Palearctic populations (1-15%), the number of colonizers could vary between 9 and 149 individuals.

Colonization of America by Drosophila subobscura: Experiment in natural populations that supports the adaptive role of chromosomal-inversion polymorphism

North America and South America have recently been colonized by the Palearctic species Drosophila subobscura. This double colonization offers a rare opportunity for evolutionary studies. Correlations between chromosomal arrangement frequencies and latitude were calculated for the colonizing populations. Signs of these correlations are highly coincident with those found in the Old World. These results provide experimental support for the adaptive value of the chromosomal-inversion polymorphism; historical and other nonadaptive explanations are thus excluded or relegated to a secondary role.

Wing-size heritability in a natural population of Drosophila subobscura

Heritability of wing size was determined in a natural population of Drosophila subobscura for two consecutive year samples. In the 1988 sample, heritability in the laboratory environment was around 15%, whereas the lower bound in nature was around 0.1%. On the other hand, in the 1989 sample, heritability in the laboratory was around 90% and in nature around 15%. Differences between the two years could be caused by the more variable climate in which the males used as fathers of the 1988 sample developed. This indicates the importance of determining the environment in which parents developed in nature before discussing the heritability values obtained.

Colonization of America by Drosophila subobscura: the founder event and a rapid expansion

The recent colonization of the American continent by Drosophila subobscura offers a unique opportunity to analyze the response of different genomic entities to a new environment. Here, results from a study of 15 allozyme loci in six North American and seven South American populations are compared with existing data from Palearctic populations. In general, only alleles with a frequency higher than 0.1 in European populations are present in America. The observed alteration in allele frequencies can be explained by a founder event. Although some significant latitudinal clines for allozyme frequencies have been detected, the results obtained are not as clear-cut as are those for chromosomal inversions. Overall heterozygosity is similar between North and South American populations and does not differ from that of Palearctic populations. This observation experimentally corroborates the results provided by theoretical models in which the average heterozygosity per locus depends not only on the size of bottleneck but also on the rate of population growth. The resemblance between North and South American populations is clearly manifested by a geometrical representation using Bhattacharyya's distance and a multidimensional scaling technique. Furthermore, these American populations are clearly differentiated from the Palearctic populations.

A photographic map of Drosophila madeirensis polytene chromosomes

A photographic map of salivary gland polytene chromosomes of Drosophila madeirensis has been constructed showing homologies and differences with respect to the standard gene arrangement of D. subobscura. Only two paracentric inversions in the X chromosome and some slight minor dissimilarities of one or two bands in the autosomes differentiate the chromosomes of these species.

Genetic analysis of extra sex combs in the hybrids between Drosophila subobscura and D. madeirensis

Drosophila subobscura and D. madeirensis are closely related species, the first distributed over a large area and the latter restricted to the island of Madeira. These species can hybridize in laboratory conditions, yielding fertile females and sterile males. Hybrid offspring show several phenotypic anomalies, including sex combs on the second and third pairs of legs in males. The extra sex comb trait is a homeotic transformation of the mesothoracic and metathoracic legs into prothoracic legs. This anomaly is observed almost exclusively in F1 males with D.madeirensis mothers. Analysis of backcross males shows that D.subobscura and D.madeirensis have diverged at a minimum of four autosomal loci affecting the extra sex comb anomaly. In addition, some incompatibility involving the X chromosome and/or a maternal effect is also implicated.

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock. III. Chromosomal-inversion and enzymatic polymorphism variation in lines selected for heat shock resistance

Two replicate selection experiments to increase and decrease heat shock resistance of Drosophila subobscura adults were carried out maintaining control lines. In the present paper, the chromosomal-inversion and enzymatic polymorphism variation with selection is analyzed. The results indicate an erratic variation of chromosomal arrangement frequencies for practically all the chromosomes in the selected lines, showing a loss of the less frequent arrangements especially in sensitive lines. Only the A chromosome and the O + 4 arrangement show a behaviour that may not be due to random effects, which points to the possible existence of heat shock factor(s) in these chromosomes. Similarly, an erratic variation of allele frequencies is observed for all the enzymes studied (Aph, Pept-1) except for the Hk-1 enzyme. We cannot establish the possible participation of this locus in heat shock resistance from the results obtained up to now. A significant decrease in heterozygosity is detected in sensitive lines from chromosomal-inversion polymorphism.

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock : 2. Modification of heat resistance by indirect selection

We have carried out two equivalent selection experiments to increase and decrease heat shock resistance of Drosophila subobscura adults, using an indirect selection method that avoids excessive consanguinity. Heat shock was 33±0.5 °C at saturation humidity. Control lines showed a rapid change of the physiological trait as a consequence of laboratory culture conditions, expressed as a decrease both in heat shock resistance and in the initial population variability for heat shock resistance. Thus, this reduction of variability seems to consist in the loss of those combinations of genes that confer high resistance to heat shock. After eight generations of selection, the selected lines were differentiated from their respective control lines, and the selection response obtained was similar in "resistant" and "sensitive" lines. Differences in survival of progeny of reciprocal crosses between selected lines suggest that inheritance of heat resistance may depend in part on the origin of egg cytoplasm.

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock : 1. Breeding temperature and crowding

Survival time following a high temperature shock of Drosophila subobscura adults in dry air has shown great variability. This experimental condition involved desiccation as the first cause of death. Here survival is studied under saturation humidity, so that the mortality may be imputed only to thermic stress. We analyze the influence of culture temperature and crowding on resistance for different sex and age of the adults. The results show strong influences of these environmental factors on heat shock resistance and show interactions with the age and sex of the adults. We suggest that these facts could be due to acclimatization and/or to adaptation. The acclimatization would occur during development and would affect physiological processes related to aging of the flies. The adaptation would take place for selection, acting through differential mortality before the heat shock. Of course, other processes could be significant. Whatever the causal explanation, it will be necessary in any future research related with heat shock resistance to take these factors into account.

Genetic changes in mating activity in laboratory strains of Drosophila subobscura

Eleven populations of Drosophila subobscura that had been maintained in laboratory conditions during different periods of time were examined for evidence of genetic divergence in mating activity. The results indicate that mating activity increases with the time of maintenance under laboratory conditions.

Effect of larval crowding on adult mating behaviour in Drosophila melanogaster

The effect of larval density on male mating success has been investigated with two strains of Drosophila melanogaster, a wild strain and a mutant strain, under low and high larval competition, and four different genotypic frequencies. The results show a strong sexual selection against mutant males when flies have been raised under low larval competition. Under high larval competition, there is a reduction in mating disadvantage of mutant males. In both instances, a frequency-dependent sexual selection exists. These results explain adequately the evolution of experimental populations where egg to adult viability and male mating success are the most important components of fitness.

Differences in chromosome A arrangement between Drosophila madeirensis and Drosophila subobscura

The proximal half of the A (= X) chromosome of D. madeirensis has a gene arrangement very similar to the A1 or A6 inversions found in D. subobscura. Polytene chromosome analysis of hybrids between D. madeirensis and strains of D. subobscura homozygous for such inversions shows, however, that D. madeirensis has a gene arrangement different from any known for D. subobscura. These results provide evidence for a greater differentiation of the X chromosome in these species than has previously been described; it seems that the X chromosome is the only one that has undergone structural variation during the speciation process.

Correlated responses to selection for wing length in allozyme systems of Drosophila melanogaster

Significant changes of genotypic structure in 20 lines selected for wing length are detected by analysis of the allelic frequencies of several enzyme loci (XDH, LAP-D, EST-6, 1-APH, ADH, α-GPDH). These changes are not haphazard but a consequence of the effects of selection on the genetic structure of the population, since replicate lines always behave in a parallel way. The changes are larger in the lines selected for short wings, in which the genetic variability decreases considerably. This decrease is the result of selection for homozygosity, detected at the allozyme loci, but most probably reflects homozygosity of more or less extended chromosomal segments. Selection for wing length, especially for short wings, favoured recombinants of the initial founder chromosomes. Only in the 1-APH and the EST-6 loci, separated by 11.7 centimorgans on the genetic map, do the alleles linked in the founder lines change in parallel in the control and long wing lines. The correlated response in the allozyme allele frequencies cannot be accounted for by a direct influence of the allozymes on the variability in wing length. The changes in the EST-6, 1-APH and perhaps in the LAP-D, can be explained by a direct effect of natural selection on the allozyme loci, probably in interaction with the effect of selection for wing length on linked loci. This last effect seems to be the main factor contributing to the change detected in the XDH locus.

Distances between populations ofDrosophila subobscura, based on chromosome arrangement frequencies

Distances between populations ofDrosophila subobscura, based on differences in the frequencies of chromosomal arrangements have been estimated using data from about 65 populations. The distances have been calculated using the formula:[Formula: see text] wherer is the number of loci or chromosomes (in the case of chromosomal polymorphism) considered,p 1jk the frequency of the allele or chromosomal arrangementk in the locus or chromosomej in the first population, andp zjk the corrsponding value in the second population.The main conclusion drawn from this analysis is that historical as well as adaptive factors are important in explaining the geographical distribution of chromosomal arrangements inD. subobscura. In general, isolated populations maintain primitive features in their chromosomal polymorphism. This is reflected in a tendency to similarity between these populations. Also, a very sharp effect of geographical barriers is detected in the distribution of the chromosomal arrangements.Two main factors are considered responsible for the strong effect that isolation has on geographical distribution of chromosome arrangements. These factors are the non-recurrence of rearrangements and the difficulty of establishing in one population the supergenes originated in another area, because of lack of coadaptation with the gene pool of the recipient population.

Inversion heterozygosity and selection for wing length in Drosophila subobscura

1. Selection for long or short wings was carried out in three pairs of lines of which two were started from the same basic population. At the end of the selection experiment the frequencies of the chromosomal arrangements were determined in each line and compared with the corresponding original frequencies to see if the chromosomal polymorphism ofDrosophila subobscurais related to the genetic variation of size in a regular way.

2. The three pairs of lines show an asymmetrical response to selection. Selection for long wings is less effective in changing the mean of the population than selection for short wings.

3. Heritability is lower in the lines selected for long wings, especially in the later generations of selection.

4. Viability shows a very small decrease during selection in most selected lines.

5. In no case does the coefficient of variability show statistically significant changes, but there is a general tendency for it to decrease.

6. Selection for long wings favours combinations heterozygous for the standard chromosome orders and specific complex inversion orders. Selection for short wings generally fixes in homozygous combination specific complex inversion orders.

7. The heterozygous combinations of the standard chromosomal orders, which are most frequent in northern populations, when combined with the complex inversion orders which are most frequent in southern populations, have a heterotic effect on size.

8. The complex inversion orders usually fixed in homozygous combinations in the lines selected for short wings are those most frequent in southern populations which have a smaller mean size than northern populations.

9. The blocks of genes integrated in ‘supergenes’ in the inverted segments of the chromosomes ofD. subobscuradiffer in genetic composition with respect to the control of wing length.