Adding the heterochromatic YL arm to an X chromosome reduces reproductive fitnesses in Drosophila melanogaster: implications for the evolution of rDNA, heterochromatin, and reproductive isolation

For X-Y exchange to be of importance in the coevolution of X and Y rDNA, there must be a mechanism to maintain cytologically normal X chromosomes in the face of continual infusions of X.YL chromosomes produced by X-Y exchanges. Replicated populations were founded with different frequencies of isogenic X and X.YL chromosomes. The X.YL chromosome declined in frequency over time in all lines. Relative fitnesses, estimated from chromosome frequency trajectories, were 0.40, 1.01, and 1.0 for X.YL/X.YL, X.YL/X, and X/X females and 0.75 and 1.0 for X.YL/Y and X/Y males, respectively. The equilibrium frequency for the X.YL chromosome due to the balance between X-Y exchange and selection was predicted to be 4-16 x 10(-4). The results strengthen the evidence for the involvement of X-Y exchange in the coevolution of X and Y rDNA arrays. Conditions for the evolution of reproductive isolation by sex-chromosome translocation are much less probable than previously supposed since the X.YL translocation chromosome is at a selective disadvantage to cytologically normal X chromosomes. Additional heterochromatin was not neutral but was only deleterious beyond a threshold, as one dose of the heterochromatic XL arm did not reduce female reproductive fitness, but two doses did.     

Extreme heterochiasmy and nascent sex chromosomes in European tree frogs

We investigated sex-specific recombination rates in Hyla arborea, a species with nascent sex chromosomes and male heterogamety. Twenty microsatellites were clustered into six linkage groups, all showing suppressed or very low recombination in males. Seven markers were sex linked, none of them showing any sign of recombination in males (r=0.00 versus 0.43 on average in females). This opposes classical models of sex chromosome evolution, which envision an initially small differential segment that progressively expands as structural changes accumulate on the Y chromosome. For autosomes, maps were more than 14 times longer in females than in males, which seems the highest ratio documented so far in vertebrates. These results support the pleiotropic model of Haldane and Huxley, according to which recombination is reduced in the heterogametic sex by general modifiers that affect recombination on the whole genome.     

Postmating reproductive isolation and modification of the ‘sex ratio’ trait inDrosophila subobscura induced by the sex chromosome gene arrangement A2+3+5+7

Drosophila subobscura males were trapped in Tunis, and mated to different lab strains. The offspring from 15% of these wild Tunisian males consisted of more than 90% females. Chromosome analysis showed that these males had carried the A2+3+5+7 which was described as 'sex ratio' chromosome, endemic in North Africa and the Canary Islands. The mean female frequency in the total offspring of all trapped males was 61%. This percentage was stable for more than ten years. F1 females from the mating of wild Tunisian males to Küsnacht standard females were backcrossed to Küsnacht standard males. In the offspring of this back cross, A2+3+5+7-males were sterile. The fertility of A2+3+5+7-males could be restored in two ways: 1) When the Küsnacht standard autosomes were replaced by Tunisian autosomes, most of the A2+3+5+7-males were again fertile. The A2+3+5+7-chromosome seems to be incompatible with autosomes from a geographically distant region. 2) After exchanging autosomes between lines, in which A2+3+5+7-males were 100% sterile, fertility could be restored in 30% of the A2+3+5+7-males. All males carrying one specific A2+3+5+7 stayed sterile as well in combination with autosomes from different lines as with Tunisian autosomes. The Y-chromosome and the cytoplasm was the same in sterile and in fertile A2+3+5+7-males. Therefore the origin of the Y-chromosome and the cytoplasm could not play a major role in sterility. The percentage of fertile males varied for different Y-chromosomes. Thus the Y-chromosomes may have some influence on fertility in this study. The restored fertility of A2+3+5+7-males can be explained assuming complementation. Defects of autosomes, and perhaps of the Y-chromosomes, could differ from line to line. Genomic changes may have happened when the A2+3+5+7 was in the genome together with autosomes and Y-chromosomes from Swiss populations. The A-chromosome which prevented fertility in all combinations, is thought to be itself defective. In one cross the 'sex ratio' trait was modified. In the offspring of some males the male to female ratio was 1:1. The variable sex ratio in the offspring from different males may have been an effect of the autosomes. In short, the intraspecific hybrid sterility and modification of the 'sex ratio' trait in D. subobscura indicate that: a) an incompatibility possibly existed between the gene arrangement A2+3+5+7 from one population and autosomes respectively Y-chromosomes from a population isolated from the former. b) In addition unidentified genomic changes occurred, c) induced by the A2+3+5+7-chromosome. d) The sex chromosomes A and Y, and the autosomes were involved.     

Studies of consomic mice bearing the Y chromosome of the BXSB mouse

Previous studies have demonstrated that the Y chromosome of the BXSB mouse can lead to accelerated autoimmunity in inbred BXSB mice and in F1 hybrids. To additionally study the effects of the BXSB-Y, we have studied three sets of Y-consomic mice, NZB.BXSB-Y, NZW.BXSB-Y, and CBA/J.BXSB-Y, each consisting of background genes from the non-BXSB parent and the Y chromosome from the BXSB mouse. The effect of the BXSB-Y on autoantibody production, immunopathology, and survival was assessed. We found that the CBA/J.BXSB-Y mice showed few differences from control CBA/J males. In contrast, NZW.BXSB-Y males had accelerated renal and cardiac disease and early death, resembling that previously reported for (NZW X BXSB)F1 mice. NZB.BXSB-Y males had accelerated anti-erythrocyte autoantibodies but not accelerated anti-DNA. They lived almost as long as NZB mice. The presence of the BXSB-Y in all of the consomic mice was confirmed by crossing the consomic mice with BXSB females and demonstrating accelerated disease in the male offspring. This study demonstrates that the BXSB-Y chromosome autoimmune accelerating factor does not act alone but operates through other genes, and that the effects on different genetic backgrounds are different. The studies have implications for human lupus; they also provide a basis for future molecular biology studies of the BXSB-Y and the genes upon which it acts.     

Human chromosomes. Evidence for autosomal sexual dimorphism.

The karyotypes of 100 males and 100 females, each assembled by the trypsin banding method, are examined in a study designed to investigate sex differences among autosomes. It is shown that female autosomes are consistently longer than those of the males, with respect to both the short and long arm measurements. In addition, discriminant analysis is used to distinguish between the male and female karyotypes. We find that, using autosomal measurements alone, this can be done with a high probability of success.     

Chromosome surface area : Further evidence for autosomal sexual dimorphism

The karyotypes of 76 males and 84 females, each assembled by the trypsin banding method, are examined in a study designed to investigate sex differences among autosomes. It is shown that female autosomes have consistently larger surface areas than the males, with respect to both the short and long arm measurements. In addition, discriminant function analysis is used to distinguish between the male and female karyotypes. We find that, using autosomal measurements alone, this can be done with a high probability of success.     

Radiation biology and inherited sterility in false codling moth (Lepidoptera: Tortricidae)

False codling moth, Cryptophlebia leucotreta (Meyrick), male and female mature pupae and newly emerged adults were treated with increasing doses of gamma radiation and either inbred or out-crossed with fertile counterparts. For newly emerged adults, there was no significant relationship between dose of radiation and insect fecundity when untreated females were mated to treated males (N female by T male). However, fecundity of treated females mated to either untreated (T female by N male) or treated males (T female by T male) declined as the dose of radiation increased. A similar trend was observed when mature pupae were treated. The dose at which 100% sterility was achieved in treated females mated to untreated males (T female by N male) for both adults and pupae was 200 Gy. In contrast, newly emerged adult males treated with 350 Gy still had a residual fertility of 5.2% when mated to untreated females, and newly emerged adult males that were treated as pupae had a residual fertility of 3.3%. Inherited effects resulting from irradiation of parental (P1) males with selected doses of radiation were recorded for the F1 generation. Decreased F1 fecundity and fertility, increased F1 mortality during development, and a significant shift in the F1 sex ratio in favor of males was observed when increasing doses of radiation were applied to the P1 males.     

Cadaveric sperm induces intergeneric androgenesis in the fish, Hemigrammus caudovittatus

Intergeneric androgenetic golden Buenos Aires tetra (BT), Hemigrammus caudovittatus was generated using sperm drawn from post-mortem males preserved at -20 degrees C for 10, 20, 30 and 40 days or fresh sperm to activate the UV-irradiated oocytes of black widow tetra (WT), Gymnocorymbus ternetzi. UV-irradiation (4.2 W/m(2)) of the oocytes for 3 min inactivated their nuclear genome. Fry hatched out from these activated oocytes were haploids; suffering haploid syndrome, they died before or within 48 h after hatching. Fresh BT sperm activated 95% oocytes; however, the sperm drawn from post-mortem males preserved at -20 degrees C for 60 (within glycerol packing) and 30 days (without glycerol packing) activated only 24 and 19% oocytes, respectively. Following activation, diploidy was restored by shocking the 25-min-old embryos at 41 degrees C for 2 min. Nuclear genomic inactivation of the oocytes was confirmed by (i) production of 100% haploids, (ii) karyotype and erythrocyte measurements, (iii) phenotypic markers, (iv) progeny testing and (v) species-specific marker. At hatching, survival of androgenotes decreased from 11% for those induced with fresh sperm to 4% for those generated using sperm from 30-day-old post-mortem males. Reproductive performance of the 'fresh' and 'cadaveric' F(0) and F(1) androgenetic males (Y(2)Y(2)) was superior to the control (X(1)Y(2)). Crosses involving homozygous (Y(2)Y(2)) 'fresh' F(0) androgenetic males with heterozygous females (X(1)X(2)) and F(0) homozygous males (Y(2)Y(2)) with females (X(2)X(2)) produced 2-4% unexpected female progenies. Paternal autosomes, inherited by the homozygous androgenetic female (X(2)X(2)), induced the production of female progenies in significantly less number of crosses than the crosses with heterozygous females (X(1)X(2)), which carried equal number of paternal and maternal autosomes. PCR analyses of the genomic DNA of normal male and unexpected F(1) and F(2) female progenies amplified by DMRT 1 specific primer produced bands of 237 and 300 bp length, and thereby confirmed that these unexpected females were genetic males. RAPD analyses of the androgenetic progenies showed that their genome was not contaminated with maternal genome.     

The Genetic Basis of Sex Ratio in Silene Alba (= S. Latifolia)

A survey of maternal families collected from natural populations showed that the sex ratio in Silene alba was slightly female biased. Sex ratio varied among populations and among families within a female biased population. Crosses among plants from the most female biased population and the most male biased population showed that the sex ratio polymorphism was inherited through or expressed in the male parent. Males from one family in particular exhibited a severe female bias, characterized by less than 20% male progeny. The inheritance of sex ratio was investigated using a reciprocal crossing design. Sex ratios from reciprocal crosses were significantly different, indicating either sex-linkage or cytoplasmic inheritance of sex ratio. The sex ratios produced by males generally resembled the sex ratios produced by their male parents, indicating that the sex ratio modifier was Y linked. The maternal parent also significantly influenced sex ratio through an interaction with the genotype of the paternal parent. Sex ratio, therefore, is apparently controlled by several loci. Although sex ratio bias in this species may be due to deleterious alleles on the Y chromosome, it is more likely to involve an interaction between loci that cause the female bias and a Y-linked locus that enhances the proportion of males in the progeny.     

Origin and evolution of mammalian sex chromosomes

Mammals present an XX/XY system of chromosomal sex determination, males being the heterogametic sex. Comparative studies of the gene content of sex chromosomes from the major groups of mammals reveal that most Y genes have X-linked homologues and that X and Y share homologous pseudoautosomal regions. These observations, together with the presence of the two homologous regions (pseudoautosomal regions) at the tips of the sex chromosomes, suggest that these chromosomes began as an ordinary pair of homologous autosomes. Birds present a ZW/ZZ system of chromosomal sex determination where females are the heterogametic sex. In this case, avian sex chromosomes are derived from different pairs of autosomes than mammals. The evolutionary pathway from the autosomal homomorphic departure to the present-day heteromorphic sex chromosomes in mammals includes suppression of X-Y recombination, differentiation of the nascent non-recombining regions, and progressive autosomal addition and attrition of the sex chromosomes. Recent results indicate that the event marking the beginning of the differentiation between the extant X and Y chromosomes occurred about 300 million years ago.     

Trypanosoma congolense: Genetic control of resistance to infection in mice

Primary isolates of Trypanosoma congolense show a range of virulence in NMRI mice. Stabiliates derived from an isolate (Dinderesso/ 80/CRTA/3) which showed moderate virulence in most NMRI mice (moderate parasitemia and survival) were used in inbred mice. C57B1/6 were resistant with low parasitemia and survival. Parasitemias were higher in males than females. BALB/c were the most sensitive of the strains tested and died with fulminating parasitemia. Inheritance of resistance, defined as low parasitemia, was studied using these two strains. Male F1 showed high parasitemia; the backcrosses of F1 to the resistant parent had a ratio of one susceptible to one resistant product; the product of F1 to susceptible parent were all susceptible; and the F2 crosses showed a ratio of three susceptible to one resistant product. The results obtained with female F1, backcrosses, and F2 mice showed similar segregation to that found using males, but the range of parasitemia was always 1–2 log₁₀ lower, except for the F1 backcrossed to BALB/c, where female and male parasitemia were undistinguishable. The segregation ratios were identical whether resistant females were crossed with sensitive males or vice-versa. The results obtained are compatible with resistance being a recessive trait controlled by a single autosomal gene (or gene cluster). In addition, sex-associated factors appear to confer higher resistance in females.     

Genetic studies of two sister species in the Drosophila melanogaster subgroup, D. yakuba and D. santomea

We performed genetic analysis of hybrid sterility and of one morphological difference (sex-comb tooth number) on D. yakuba and D. santomea, the former species widespread in Africa and the latter endemic to the oceanic island of S?o Tomé, on which there is a hybrid zone. The sterility of hybrid males is due to at least three genes on the X chromosome and at least one on the Y, with the cytoplasm and large sections of the autosomes having no effect. F1 hybrid females carrying two X chromosomes from either species are perfectly fertile despite their genetic similarity to completely sterile F1 hybrid males. This implies that the appearance of Haldane's rule in this cross is at least partially due to the faster accumulation of genes causing male than female sterility. The larger effects of the X and Y chromosomes than of the autosomes, however, also suggest that the genes causing male sterility are recessive in hybrids. Some female sterility is also seen in interspecific crosses, but this does not occur between all strains. This is seen in pure-species females inseminated by heterospecific males (probably reflecting incompatibility between the sperm of one species and the female reproductive tract of the other) as well as in inseminated F1 and backcross females, probably reflecting genetically based incompatibilities in hybrids that affect the reproductive system. The latter 'innate' sterility appears to involve deleterious interactions between D. santomea chromosomes and D. yakuba cytoplasm. The difference in male sex-comb tooth number appears to involve fairly large effects of the X chromosome. We discuss the striking evolutionary parallels in the genetic basis of sterility, in the nature of sexual isolation, and in morphological differences between the D. santomea/D. yakuba divergence and two other speciation events in the D. melanogaster subgroup involving island colonization.     

Field survey of sex-reversals in the medaka, Oryzias latipes: genotypic sexing of wild populations

The medaka, Oryzias latipes, has an XX/XY sex determination mechanism. A Y-linked DM domain gene, DMY, has been isolated by positional cloning as a prime candidate for the sex-determining gene. Furthermore, the crucial role of DMY during male development was established by studying two wild-derived XY female mutants. In this study, to find new DMY and sex-determination related gene mutations, we conducted a broad survey of the genotypic sex (DMY-negative or DMY-positive) of wild fish. We examined 2274 wild-caught fish from 40 localities throughout Japan, and 730 fish from 69 wild stocks from Japan, Korea, China, and Taiwan. The phenotypic sex type agreed with the genotypic sex of most fish, while 26 DMY-positive (XY) females and 15 DMY-negative (XX) males were found from 13 and 8 localities, respectively. Sixteen XY sex-reversals from 11 localities were mated with XY males of inbred strains, and the genotypic and phenotypic sexes of the F(1) progeny were analyzed. All these XY sex-reversals produced XY females in the F(1) generation, and all F(1) XY females had the maternal Y chromosome. These results show that DMY is a common sex-determining gene in wild populations of O. latipes and that all XY sex-reversals investigated had a DMY or DMY-linked gene mutation.     

Segregation distortion of X-linked marker genes in interspecific crosses between Mus musculus and M. spretus

An interspecific cross was made between females of the C3H/HeHa.Pgk-1 a inbred laboratory strain of Mus musculus and males of the separate species Mus spretus. The F1 males are sterile but the F1 females are fertile and they were backcrossed to both C3H and spretus males. Evidence is presented from the segregation of X-linked marker genes that the interspecific F1 female has a genetically deleterious effect on the C3H X chromosome that is expressed as a male-lethal effect with the spretus Y chromosome but not with the musculus Y chromosome of C3H.     

Genetics of hybridization of Glossina swynnertoni with Glossina morsitans morsitans and Glossina morsitans centralis

Abstract Reciprocal crosses were performed with Glossina swynnertoni and Glossina morsitans morsitans and with G. swynnertoni and Glossina morsitans centralis , using strains that carried marker genes in all three linkage groups. Glossina swynnertoni males can inseminate, but not fertilize, G.m.morsitans; all other crosses produced some fertile females. Hybridization did not cause sex ratio distortion among F_{ flies. Most F and backcross females were fertile, but all F, males were sterile. Sterility among backcross males was also high (99% in BXj, 85% in Bx₂, and about 50% in Bx₃ to Bx₅). Chromosome transmission by hybrid females usually conformed to Mendelian expectations, but genetic recombination was lower than observed in G.m.morsitans. The reduction in fertility among backcross females was not associated with heterozygosity in any linkage group. Sterility among hybrid and backcross males was associated with heterozygosity of sex chromosomes and probably autosomes. The results support the systematic placement of G.swynnertoni closer to G.mxentralis than to G.m.morsitans.     

Evolution of nesting strategies of ants: genetic evidence from different population types of Formica ants

Genetic population structure was studied in two types of populations in the ants Formica exsecta and F. pressilabris: populations consisting of single-nest colonies (monodomy) and populations consisting of multi-nest colonies (polydomy). These characteristics seem to be associated with the number of egg-laying females (gynes) in a nest, mating structure of the population, sex ratio and male size variation. The monodomous populations are characterized by single-gyne nests, the population sex ratio is either I:1 or female-biased, males are mainly large-sized, and there is slight inbreeding in the population. The polydomous populations have multi-gyne nests with gynes related to each other, sex ratio is strongly male-biased, most males are small-sized, and there is slight genetic microdifferentiation within the populations. Diploid males found in a polydomous F. pressilabris population suggest that the population is inbred and isolated. Habitat localization is presented as a plausible explanation for the evolution of the polygynous and polydomous population structure.     

Methyl eugenol effects on Bactrocera dorsalis male total body protein,reproductive organs and ejaculate

Methyl eugenol (ME) and inclusion of protein into the adult diet increase the mating competitiveness of the Oriental fruit fly, Bactrocera dorsalis (Hendel). Exposing males to ME or protein is a promising post‐teneral treatment for males being released in the sterile insect technique (SIT). However, the effect of this post‐teneral treatment on male reproductive organs or the male ejaculate is unknown. During mating, males transfer sperm and accessory gland products (AGPs) to females and these compounds are reported to modulate female sexual inhibition. We studied the impact of male exposure to ME and a yeast hydrolysate (YH) diet on the protein reserves of males, male reproductive organ size, and the male ejaculate through sperm and AGPs. We show that males exposed to ME regardless of access to YH accumulated a greater amount of whole body protein. Males fed on YH also had increased total body protein and had bigger reproductive organs than YH‐deprived males, but no apparent effect of ME exposure was observed on reproductive organ size. Females stored less sperm when mated with males fed on YH and ME compared to males not fed on ME. YH and ME had no effect on male AGPs. Females injected with AGPs of males fed on YH and exposed to ME were just as likely to mate as females injected with AGPs of non‐treated males. However, females injected with AGPs of males exposed to ME mated faster than females injected with AGPs of non‐exposed males. We conclude that while exposure to ME increases male copulatory success and protein reserves in the male body, there seem to be some potential trade‐offs such as lower sperm stored by females. We discuss our results in terms of pre‐release protocols that may be used for B. dorsalis in SIT application.     

Nonpairing of the X and Y chromosomes in the spermtocytes of BDF1 mice

In the hybrid mouse strain BDF1, some 35--40% of spermatocytes had unpaired X and Y choromosomes in stages ranging from diplotene to first meiotic metaphase. This phenomenon varied significantly from mouse to mouse. In pooled material from Swiss Albino and CF1 mice, the corresponding frequency was 5.7%. In C57 BL/6 mice, one of the parent strains of BDF1 mice, the X and Y were separate in 7.7% of the spermatocytes. Based on the behavior of the X and Y in the BDF1 strain, it is concluded that they do not pair, rather than initially pairing and then precociously separating. The factor interfering with the pairing of the X and Y does not affect the autosomes; possibly it is an incompatibility of the two sex chromosomes, which come from different inbred lines.