Translocations induced by fast neutrons and X-rays in Delia antiqua

Summary A comparison was made using X-rays and fast neutrons for the induction of translocations in Delia antiqua. Using the same radiation dose, no difference in efficiency between the two radiation types could be observed. However, with fast neutrons many multiple translocations were induced, including a quadruple translocation involving 4 out of 5 autosomes. One male linked translocation was also induced.The reciprocal translocations were assigned into two classes: symmetrical and asymmetrical, and ten of the latter were chosen for inbreeding to produce homozygotes. Asymmetrical exchanges were chosen so that translocation homozygotes could be differentiated cytologically from the normal karyotype. In seven different translocations, homozygous larvae were observed, but often at a low frequency. In four of these lines, viable adult homozygotes were observed. Subsequent random sib-crossing failed to produce a homozygous line.     

C-banding analysis on wild Emmer (Triticum dicoccoides Körn) strains with and without spontaneous reciprocal translocations

C-banding polymorphism was analyzed in eight strains of wild Emmer, Triticum dicoccoides Körn, which included six translocation homozygotes reported previously. Polymorphisms were detected in all of the strains examined, and the breakpoints of five spontaneous translocations were successfully identified by C-bands. Of the eight breakpoints that could be precisely identified, one was located in the centromeric region while the remaining seven were located in proximal to distal euchromatic regions. The two breakpoints of one translocation could only be approximately localized to proximal regions due to the scarcity of C-bands. The present results are in contrast with those observed on T. araraticum, another wild tetraploid wheat belonging to the Timopheevi group, in which most of the breakpoints were located in centromeric regions. In T. dicoccoides, the six translocation chromosome types were derived from the standard karyotype primarily by a mechanism other than centric breakage-fusion.     

VARIATION IN THE STRENGTH AND SOFTNESS OF SELECTION ON DELETERIOUS MUTATIONS

Deleterious alleles constantly enter populations through mutation. Understanding the nature of selection against such alleles is required to assess their impact on populations. In a subdivided population, two distinct aspects of selection are important: the strength and softness of selection. Using Drosophila melanogaster, we estimated both aspects of selection for each of eight loci across two environments. These data allow us to test conflicting predictions about the factors affecting the softness of selection. First, we show that the softness of selection is not determined by ecological conditions alone. Second, we find that resource limitation makes selection stronger but does not make it softer. Third, we find that wild‐type individuals tend to benefit more than mutants from being reared with competitors of low genetic quality. This means that selection is effectively “harder” on mutants than wild types. A model is presented showing that the sensitivities of mutants and wild types to local competitors differentially affect equilibrium mutation frequency and measures of load.     

Geographic variation of sex-linked translocation heterozygosity in the termite Kalotermes approximatus snyder (Insecta: Isoptera)

The primitive termite Kalotermes approximatus carries a number of reciprocal translocations (segmental interchanges) that are linked to the sex-determining mechanism in such a way that males are permanent structural heterozygotes, forming long chains or rings of chromosomes in meiosis, while females are structural homozygotes, forming only bivalents. A survey of male meiosis from collections covering nearly the whole species range in the southeastern United States reveals considerable variation in the number of translocations: males with a diploid number of 32 or 33 have meiotic chains of 11, 13, 14, 15, 16, and 17 or 19 chromosomes. The different types can be arranged in an evolutionary series of rearrangements involving translocations or Robertsonian fusions between chromosomal elements in the ring and those outside. In addition, the existence of a closed chain (ring) of 16, and of four different types of chain of 13, indicate that similar rearrangements have occurred among chain elements. The geographic pattern of these rearrangements suggests that their selection accompanied the expansion of the species northward from southern Florida sometime since the last glaciation or, alternatively, that as they arose the new translocation types successively supplanted the ancestral types, preferentially in the east-central portion of the range.     

Translocations and a balanced polymorphism in a Drosophila population

A Drosophila population cage initiated with equal numbers of two viable II–III translocation homozygotes rapidly evolved into a balanced polymorphism with the two translocations maintained throughout 25 generations at which time the experiment was terminated. The fertility of this population averaged 26%; a control population averaged 90%. The establishment of the polymorphism was interpreted with reference to the reduced viability of the two homozygotes such that their net fitness was considerably less than that of partially sterile double heterozygote. By the incorporation of specific values for the relative fitness of the three genotypes in a computer programme it was possible to simulate the polymorphism.     

Genetic background damage accompanying reciprocal translocations induced by X-rays and fission neutrons in Arabidopsis and Secale

Translocations were induced in Arabidopsis and rye by fission neutrons and X-rays using doses with equal effects. Segregations of these translocations were studied in M₃, M₄ and S₁ of backcrosses. When there was a deficit of non-translocation homozygotes it was concluded that genetic damage had occurred in chromosomes homologous with the translocation chromosomes. The frequency of families with shortage (including absence) of translocation homozygotes was much larger than families with background damage. This demonstrates that reduced viability in the former was due to true breakpoint damage and not to linked damage. The frequency of translocation breakpoint damage was the same for X-rays and fission neutrons, but the latter probably induced more serious damage. Background damage was also the same for the two types of radiation. Therefore, neutrons should not be considered a ‘cleaner’ inducer of translocation than X-rays.     

Reciprocal translocation of small numbers of inbred individuals rescues immunogenetic diversity

Genetic rescue can reduce inbreeding depression and increase fitness of small populations, even when the donor populations are highly inbred. In a recent experiment involving two inbred island populations of the New Zealand South Island robin, Petroica australis, reciprocal translocations improved microsatellite diversity and individual fitness. While microsatellite loci may reflect patterns of genome‐wide diversity, they generally do not indicate the specific genetic regions responsible for increased fitness. We tested the effectiveness of this reciprocal translocation for rescuing diversity of two immunogenetic regions: Toll‐like receptor (TLR) and major histocompatibility complex (MHC) genes. We found that the relatively small number of migrants (seven and ten per island) effectively brought the characteristic TLR gene diversity of each source population into the recipient population. However, when migrants transmitted TLR alleles that were already present at high frequency in the recipient population, it was possible for offspring of mixed heritage to have decreased gene diversity compared to recipient population diversity prior to translocation. In contrast to TLRs, we did not observe substantial changes in MHC allelic diversity following translocation, with limited evidence of a decrease in differentiation, perhaps because most MHC alleles were observed at both sites prior to the translocation. Overall, we conclude that small numbers of migrants may successfully restore the diversity of immunogenetic loci with few alleles, but that translocating larger numbers of animals would provide additional opportunity for the genetic rescue of highly polymorphic immunity regions, such as the MHC, even when the source population is inbred.     

The induction by X-rays of chromosome aberrations in male guinea-pigs, golden hamsters and rabbits. II. Properties of translocations induced in post-meiotic stages.

Translocations induced by X-rays in post-meiotic germ cells of male guinea-pigs, golden hamsters and rabbits were studied cytologically in the F1 sons of the irradiated males. The percentage of spermatocytes displaying multivalent configurations varied with the translocation, but the average percentage appeared to depend on the species: fewer quadrivalents were observed in hamster than in guinea-pig heterozygotes and most were recorded for rabbit heterozygotes. Chain quadrivalents were more abundant than ring quadrivalents at meiosis for the guinea-pig and hamster, in contrast to the mouse. Too few translocation heterozygotes were examined to determine which meiotic configuration was the more prevalent in the rabbit. In all three species, as in the mouse, translocations were found which caused male sterility, due to partial or complete failure of spermatogenesis, although most translocations caused semi-sterility. For these semi-sterile males both the frequency and time of embryonic death in the progeny appeared to be the same as in the mouse. It is concluded that similar types of chromosome aberrations are induced by X-rays in post-meiotic germ cells of male guinea-pigs, rabbits, golden hamsters and mice.     

Disordered Translocation is Hastening Local Extinction of the Chinese Giant Salamander

Biodiversity is declining globally by an unprecedented extinction rate. This is especially true for amphibians, accounting for 24.3% of all threatened vertebrates. As the largest extant amphibian species in the world, wild populations of the Chinese giant salamander(Genus Andrias)(CGS) have decreased dramatically because of overexploitation and habitat degradation. Translocation has become an important strategy for restoring threatened wild populations worldwide. However, disordered tra nsloca tion usually has negative effects on the native populations.We provide an overview of CGS translocation and show that disordered translocation can increase local population extinction. Nearly four times the estimated number of wild individuals have been released across China. There a re three types of translocation used for CGS, namely, reinforcement,reintroduction and ecological replacement, the last of which accounts for over one-third of translocations.Our genetic screening revealed that most released individuals were not from local populations, with one to four lineages detected in every release site(n = 6).This disordered translocation can potentially reduce the genetic integrity of original populations. Hence,we suggest suspending current CGS translocation activities immediately, until more robust measures can be developed and implemented to improve the current translocation program, especially with respect to lineage identifica tion a nd the identifica tion of appropriate release sites.     

An alternative method for isolating homozygotes of autosomal translocations in the mosquito Culex tarsalis.

An alternative pseudolinkage procedure for isolating homozygotes of autosomal translocations has been developed with the mosquito Culex tarsalis (Coquillet). The first step was to induce a translocation heterozygote in a population that was marked with recessive mutants. Interbred translocation heterozygotes produced translocation homozygotes that were phenotypically different from their translocation heterozygote and normal siblings. Thus, a translocation homozygote line of this species was selected and established in shorter time and with less effort than by prior pseudolinkage procedure.     

EFFECTIVENESS OF SELECTION IN REDUCING THE GENETIC LOAD IN POPULATIONS OF PEROMYSCUS POLIONOTUS DURING GENERATIONS OF INBREEDING

It has been hypothesized that natural selection reduces the “genetic load” of deleterious alleles from populations that inbreed during bottlenecks, thereby ameliorating impacts of future inbreeding. We tested the efficiency with which natural selection purges deleterious alleles from three subspecies of Peromyscus polionotus during 10 generations of laboratory inbreeding by monitoring pairing success, litter size, viability, and growth in 3604 litters produced from 3058 pairs. In P. p. subgriseus, there was no reduction across generations in inbreeding depression in any of the fitness components. Strongly deleterious recessive alleles may have been removed previously during episodes of local inbreeding in the wild, and the residual genetic load in this population was not further reduced by selection in the lab. In P. p. rhoadsi, four of seven fitness components did show a reduction of the genetic load with continued inbreeding. The average reduction in the genetic load was as expected if inbreeding depression in this population is caused by highly deleterious recessive alleles that are efficiently removed by selection. For P. p. leucocephalus a population that experiences periodic bottlenecks in the wild, the effect of further inbreeding in the laboratory was to exacerbate rather than reduce the genetic load. Recessive deleterious alleles may have been removed from this population during repeated bottlenecks in the wild; the population may be close to a threshold level of heterozygosity below which fitness declines rapidly. Thus, the effects of selection on inbreeding depression varied substantially among populations, perhaps due to different histories of inbreeding and selection.     

Chromosomal polymorphism in the house mouse (Mus domesticus) of Greece and Yugoslavia

A total of 88 wild mice from the Dalmatian coast of Yugoslavia (35 animals), and Peloponnesus (30 animals) and Thebes (23 animals) on mainland Greece were karyotyped. In all but five animals Robertsonian translocations were found. Mice from the Dalmatian region were homozygous for translocations Rb(5.15), Rb(6.12), Rb(8.17), Rb(9.13), and Rb(10.14); they were homo-or heterozygous for the translocation Rb(1.11). Some of them lacked the Rb(1.11) translocation altogether so that the diploid numbers in the Yugoslavian mice were 2n=28, 29, 30, or 40. The mice from the vicinity of Olympia in northwestern Peloponnesus were homozygous for eight Robertsonian translocations: Rb(1.3), Rb(2.5), Rb(4.6), Rb(8.12), Rb(9.16), Rb(10.14), Rb(11.17), and Rb(13.15). Their diploid chromosome number was therefore 2n=24. Mice from the vicinity of Patras in northwest Peloponnesus carried all except the first three of these eight translocations; their chromosome number was 2n=30. Finally, the mice from Thebes were homozygous for translocations Rb(2.15), Rb(4.14), Rb(5.12), and Rb(10.13). They were homo- or heterozygous for Rb(6.9), Rb(8.17), and Rb(1.11); some mice lacked the Tb(1.11) translocation altogether. The translocations Rb(6.9)40Tu and Rb(10.13)42Tu represent new arm combinations not found previously in any wild mouse population. the remaining translocations have previously been found in different Mediterranean countries, in Scotland and in southern Germany. The findings suggest that each translocation arose only once and that different translocations have come together in different populations to generate a unique karyotype characterizing this population.     

Beyond the beneficial effects of translocations as an effective tool for the genetic restoration of isolated populations

Translocations are becoming increasingly popular as appropriate management strategies for the genetic restoration of endangered species and populations. Although a few studies have shown that the introduction of novel alleles has reversed the detrimental effects of inbreeding over the short-term (i.e., genetic rescue), it is not clear how effective such translocations are for both maintaining neutral variation that may be adaptive in the future (i.e., genetic restoration) and increasing population viability over the long-term. In addition, scientists have expressed concerns regarding the potential genetic swamping of locally adapted populations, which may eliminate significant components of genetic diversity through the replacement of the target population by the source individuals used for translocations. Here we show that bird translocations into a wild population of greater prairie-chickens (Tympanuchus cupido pinnatus) in southeastern Illinois were effective in both removing detrimental variation associated with inbreeding depression as well as restoring neutral genetic variation to historical levels. Furthermore, we found that although translocations resulted in immediate increases in fitness, the demographic recovery and long-term viability of the population appears to be limited by the availability of suitable habitat. Our results demonstrate that although translocations can be effective management tools for the genetic restoration of wild populations on the verge of extinction, their long-term viability may not be guaranteed unless the initial conditions that led to most species declines (e.g., habitat loss) are reversed.     

The cytogenetic systems of grasshoppers and locusts

Two new cases of supernumerary heterochromatic segments are described. One of these, found in a heterozygous state in eight males of Chorthippus jucundus, is present terminally on the short arm of the longest autosome. The other, present in a single male heterozygote of Trimerotropis tolteca modesta, is again terminally sited, this time on the long arm of the second smallest auto-some. Both segments are considered to be products of tandem duplication rather than translocation. The segment system of C. jucundus, like that of a number of other categories of heterochromatic material, produces a striking increase in mean cell chiasma frequency compared to basic homozygotes from the same population. It is argued that this effect of the segment on recombination provides a potential basis for selection leading to altered fitness. This, in turn, may well have determined the evolution of at least some forms of supernumerary material in natural populations.     

Swift action increases the success of population reinforcement for a declining prairie grouse

Translocations have become an increasingly valuable tool for conservation in recent years, but assessing the successfulness of translocations and identifying factors that contribute to their success continue to challenge biologists. As a unique class of translocation, population reinforcements have received relatively little attention despite representing a substantial portion of translocation programs. Here, we conducted population viability analyses to quantify the effects of 216 reinforcement scenarios on the long‐term viability of four populations of Greater Prairie‐Chickens (Tympanuchus cupido pinnatus) in Wisconsin, USA, and used multiple linear regression to identify factors that had the greatest relative influence on population viability. We considered reinforcements from outside of the study area in addition to translocations among Wisconsin populations. We observed the largest decreases in site‐specific extinction probability and the largest increases in the number of sites persisting for 50 years when more vulnerable populations were targeted for reinforcement. Conversely, reinforcing the most stable sites caused the greatest reduction in regional extinction probability. We found that the number of translocated hens was a comparatively poor predictor of changes in long‐term population viability, whereas the earlier onset of reinforcement was consistently associated with the greatest increases in viability. Our results highlight the value of evaluating alternative reinforcement strategies a priori and considering the effects of reinforcement on metrics of long‐term population persistence.     

Meiotic chromosomal aberrations in wild populations of Podophyllum peltatum

Meiotic chromosomal aberrations observed in wild populations of the plant Podophyllum peltatum include incomplete homologous pairing, non-homologous pairing, and inversion heterozygosity in pachytene; univalents, asymmetrical bivalents, and translocation heterozygosity in metaphase-I; bridge and fragments in anaphase-I; and non-disjunction as detected in anaphase-II. Incomplete homologous pachytene pairing is believed to result in non-homologous pairing and in the formation of metaphase-I univalents. The unequal distribution and precocious division of univalents in anaphase-I leads to non-disjunction. Non-disjunction chromosomes (varying in frequency from 0.0 to 24.6%) appear to be distributed among the genome on the basis of chromosome length. Asymmetrical bivalents and anaphase-I side-arm bridges are believed to be caused by chromatid breakage and fusion rather than inversion heterozygosity. Of the 135 clones examined, 20 were found to be heterozygous for translocations. The possibility of widespread distribution of some translocations is suggested.     

Contrasting Patterns of Nucleotide Sequence Variation at the Glucose Dehydrogenase (Gld) Locus in Different Populations of Drosophila Melanogaster

We have analyzed nucleotide sequence variation at the Glucose dehydrogenase (Gld) locus from four populations of Drosophila melanogaster from four continents. All four population samples show a significant reduction in silent variation compared to the neutral expectation. The levels of silent variation across all four populations are consistent with the predictions of the background selection model; however, Zimbabwe has a remarkably low level of variation. In the face of dramatically reduced silent polymorphism, an amino acid variant, leading to the common allozyme polymorphism at Gld, remains in low to intermediate frequency in all non-African samples. In the Chinese population sample, the ratio of replacement to silent variation is significantly elevated compared to the neutral expectation. The difference in patterns of variation across these population samples suggests that selection on Gld (or the Gld region) has been different in the Chinese population than in the other three.     

Examination of Rates and Spectrums of Robertsonian Translocations in the General Population and in Patients with Reproductive Disorders

Robertsonian translocations (rob) are essential in the etiology of congenital malformations and reproductive disorders. However, to date, there has been no systematic comparative analysis of both the frequency and spectrum of rob in different carrier populations. Also, due attention was not paid to the phenomenon of the exceptional rarity of some types of rob. In this paper, the spectrum of rob was determined in the reference group (newborn carriers from the general population and carriers diagnosed prenatally for indications other than family rearrangement) in comparison with groups of carriers with reproductive disorders (infertility and pregnancy loss). The overall rates of rob in the studied groups were calculated as 1.05, 3.3, and 5‰, respectively. It was suggested that homologous translocations do not cause disturbances in spermatogenesis. A rarity of some certain types of translocation could be explained by selection.     

Chromosome translocations in wild populations of tetraploid emmer wheat in Israel and Turkey

Translocation frequencies (as compared to the standard chromosome arrangement typified by that in Chinese Spring) in 9 or more genotypes from each of 15 populations of Triticum dicoccoides in Israel were determined. Data also were obtained from 2 genotypes of the southernmost population (Jaba). A single population from Turkey was also investigated. There were 119 genotypes with translocations in the sample of 171 genotypes investigated (70%). The frequency of translocations in different populations varied from 0.27 to 1.00, and all populations had 1 or more genotypes with one or more translocations. Some populations such as Qazrin appeared to be homogeneous for translocations, but most populations were heterogeneous. A sample of 17 genotypes from 12 of the populations were crossed with the Langdon D-genome disomic substitutions to determine the identity of the chromosomes involved in the translocations. There were nine genotypes with translocations and with the exception of a 2A/2B translocation, none of them involved the same chromosomes. The B-genome chromosomes were involved in translocations more frequently than the A-genome chromosomes. Translocation frequencies (TF) of the various populations were correlated with environmental variables, primarily with water availability and humidity, and possibly also with soil type. In general, TF was higher in peripheral populations in the ecologically heterogeneous frontiers of species distribution than in the central populations located in the catchment area of the upper Jordan valley.     

Genetic load and viability variation in korean natural populations ofDrosophila melanogaster

Summary Natural populations of Drosophila melanogaster from Anyang and Susac (suburbs of Seoul) have been analyzed with respect to viability variation on the second chromosome. Homozygotes as well as random heterozygotes for wild chromosomes were studied. The frequency of lethal factors was about 16 per cent, that of drastics 26 per cent. The average viability of homozygotes was 0.650 including lethal lines and 0.858 for quasinormals; that for random heterozygotes was 1.125. Allelism tests have been performed for the lethals. The allelism rate turned out to be as high as 0.036 and 0.0214, respectively. Using a formula by Nei, the effective population size can be estimated from these data. Korean D. melanogaster populations proved as small as 2000 to 3000 individuals. No correlation between homozygous and heterozygous viabilities could be found. According to these observations, along with the fact that partly big clusters of identic lethals could be found in the allelism tests, it is concluded that in Korean populations quite a large part of the hard genetic load is balanced. The connection between population size, population structure and associative or genuine overdominance is discussed.