CONTINUING STUDIES ON THE SOUTH AMHERST DROSOPHILA MELANOGASTER NATURAL POPULATION DURING THE 1970'S AND 1980'S

Continuing investigations on the South Amherst Drosophila melanogaster natural population following the significant decline and recovery of lethal (le) and semilethal (sle) frequencies in the late 1960's (Ives, 1970) show that the population has been remarkably stable although it contains MR (male recombination) and/or P DNA elements (Kidwell et al., 1977a; Green, 1980). A 13-year study affirms that the lethals present are nonrandomly distributed along the second chromosome and deficient on the right; they differ significantly in distribution from spontaneous (Ives, 1973) and δ-induced lethals (Minamori and Ito, 1971). Between 1970 and 1977, a total of 4,083 second chromosomes from the Markert subpopulation were analyzed; 28.9% of the chromosomes were lethal and 7.25% were semilethal in homozygous condition. Frequencies are similar for early summer and late fall collections although the rate of allelism among lethals is significantly higher in early summer than in late fall. For the large fall (1970–1979) Porch site population, 2,519 second chromosomes were analyzed; 29.5% were lethal and 8.0% were sublethal as homozygotes; the rate of allelism among lethals was 1.50%. At Hockanum, 1977–1983, lethal and semilethal frequencies were lower; the rate of allelism among lethals was 1.43%. The chromosome map distribution of lethals does not change between summer and late fall at Markert. The overall distributions of lethals at the Markert and Hockanum sites are similar. In tests for male recombination (MR) activity in the population over a 6-year period, a total of 0.47% recombinants were observed; these were uniformly distributed along the second chromosome. Comparisons are made with other long studied D. melanogaster populations.     

Genetic rescue of lethal genotypes in the mouse

Deletions of gene sequences in chromosome 7 of the mouse are known to interfere with biochemical and cellular development differentiation with lethal effects in homozygotes. The presence of the corresponding wild-type alleles in Cattanach's translocation (chromosomes 7 to X) is able to “rescue” potentially lethal females if they are made heterozygous for the translocation-carrying X chromosome. This holds true for those chromosome 7 deletions with perinatally lethal effects, whereas “rescue” is not readily accomplished with the deletions that cause early embryonic lethality. Females homozygous for the relevant deletion sequences and heterozygous for the translocation-carrying X chromosome are mosaics of two cell types: those in which the wild-type alleles included in the translocated piece complement the depleted sequences, resulting in a normal cellular phenotype, and those with the ordinary X chromosome expressing the lethal phenotype. The developmental interactions between the two cell types and their role in the mechanisms responsible for survival of females homozygous for lethal deletions are discussed. The failure of “rescue” of embryonic lethals reflects as yet unknown temporal and functional aspects of X-inactivation early embryogenesis.     

Linkage and the elimination of deleterious mutant genes from experimental populations

The behavior of a balanced lethal system and its two component loci was examined in laboratory populations of the flour beetle,Tribelium castaneum. The two lethal genes wereShort antenna (Sa), a dominant morphological mutant which is a recessive lethal when homozygous, and a lethal (l) located 1 to 2 map units fromSa in linkage group VII.Frequencies of both lethals declined rapidly when present individually in populations. The elimination curve forl closely paralleled the theoretical curve for a recessive lethal;Sa was eliminated more rapidly, indicating that heterozygotes also suffer a reduced fitness relative to wild-type homozygotes. These observations were corroborated by measurements of several components of fitness.When populations were initiated with the two lethals in repulsion phase, marked differences were observed in the rate of elimination ofSa in six replicate populations. Since the initial phase of elimination depends on the occurrence of an effective crossover between the two loci, it was possible to apply a correction factor so that the observed elimination curve closely approximated a theoretical curve generated using parameters obtained from the assays of fitness components.It is argued that many cases of apparently intermediate gene frequency equilibrium reported in studies withTribolium and also withDrosophila result from linkage disequilibrium rather than overdominance. The data presented provide a striking example of the effects of linkage on the elimination of mutant genes from experimental populations.     

Genetic load and viability studies in Korean natural populations of Drosophila melanogaster

Further studies about the amount of genetic load in the Korean Anyang and Cheju (Sughipo) Island populations of Drosophila melanogaster were performed. In total 1630 second chromosomes were extracted from the Anyang opulation between 1983 and 1985; 19.0 % of the chromosomes proved lethal, 8.8 % semilethal in homozygous condition. From the island oulation, 504 wild second chromosomes were analysed in 1986; 24.2 % were lethal, 4.6 % subleiaf A slight increase of lethal and semilethal frequencies between 1976 and 1986 can be observed. Mean viabilities of “all homozy-gotes”, “quasinormal-homozygotes”, and “random heterozygotes” were estimated from crossin experiments with marker strains. Random heterozygotes were always more viable than quasinormal homozygotes. An analysis for correlation between random heterozgous and homozygous viabilities gave values significantly different from zero only for the 1985 kyang sample (r =—0.4625, P < 0.01), but no significances could be observed for all other Anyang samples from 1983, 1984, and 1986, respectively. he effective population sizes were estimated to be between 2000 and 6300 individuals for the Anyan and 4200 individuals for the island population, using Nel's formula (1968). It is sugested that baknced natural selection is mainly responsible for the maintenance of genetic load in the Anyang natural populations of D. melanogaster.     

Wild O chromosomes of Drosophila subobscura from different geographic regions have different effects on viability

By means of the marker strain Va/Ba wild chromosomes O of Drosophila subobscura were extracted from eight natural populations situated on a north-south gradient from Sweden or Scotland to Tunesia. Lethal frequencies and viability effects of the wild chromosomes O were studied in homozygous and random heterozygous combinations. In accordance with results from other Drosophila species random heterozygotes were always more viable than homozygotes. The viability-determining polygene system proved, however, dominant to some degree. Geographic differences became apparent especially with respect to three different characteristics: (1) The lethal frequencies for chromosomes O from central populations are higher than for those from northern and southern marginal populations; (2) Mean viabilities of non-lethal homozygotes and random heterozygotes are lower for central than for marginal populations; (3) The increase of viability through heterozygosity is more pronounced in the northern populations than in the others. The differences are thought to be mainly due to differences in the adaptation strategy of marginal and central populations. The viability fitness components seem of more importance for the marginal populations while fertility components may be of greater weight under central conditions. The geographic variability of the viability polygene system is finally compared with that of other genetic traits in D. subobscura.     

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.     

The genetic conditions in heterozygous and homozygous populations of Drosophila. II. X-ray induced lethals in a homozygous and a heterozygous population

Strains of Drosophila melanogaster were made isogenic for their second chromosomes by means of the marker strain LCy/Pm. One of these strains was used as a founder for a homozygous experimental population (W). All other strains were mixed and established a heterozygous population (LKW). Both populations were free of lethals in the beginning with respect to their second chromosomes. After they had been exposed to an X-ray irradiation of 7000 r they contained about 26 per cent newly induced lethal chromosomes. Whereas in the heterozygous population the lethal frequency decreased rather fast to 10 per cent, that of the homozygous population remained rather constant at 25 per cent during a period of 135 days. After a year of continuation, however, both populations reached the same lethal frequency of about 10 per cent. Allelism tests carried out after 10 generations revealed that there was a highly heterotic lethal factor in the homozygous population. After excluding this heterotic lethal from the calculations, the lethal frequencies of the two populations remained significantly different. It was assumed that the relative mean fitness of lethal heterozygotes was generally higher in the homo-than in the heterozygous populations. The results indicate that homozygous populations are much more capable of incorporation new mutations than heterozygous.     

Accumulation of Deleterious Genes in a Cage Population of DROSOPHILA MELANOGASTER

Lethal and sterility mutations were accumulated in a cage population which was initiated with lethal- and sterility-free second chromosomes of D. melanogaster. It took about 2,000 days for the frequencies of these genes to reach equilibrium levels, i.e., 18% lethal and 9% male-sterile chromosomes. Two other cage populations which were initiated with random chromosomes sampled from natural populations and kept for more than eleven years in the laboratory showed 19-20% lethal content. The elimination rates of lethals by homozygosis in these populations were smaller than the mutation rate. By using NEI's formulae, the deleterious effect of a lethal gene in heterozygous condition (h) was estimated to be 0.035. The effective population number in the cage populations was estimated to be 1,000-2,900, while the actual population number was 3,500-7,800.     

I−R hybrid dysgenesis in Drosophila melanogaster: nature and site specificity of induced recessive lethals

This paper presents results of the genetic and cytological analysis of 144 sex-linked recessive lethals, plus 1 non-lethal. All of them were induced by I−R hybrid dysgenesis. This collection of mutants was pooled from experiments involving inducer chromosomes that differ in the chrosomal position of their I elements. Our results show that 30% of the recessive lethals are associated with chromosomal rearrangements which depend on the strength of the I−R interaction. These lethals are induced on both inducer- and reactive-origin chromosomes, and their frequency is dependent on the structure of the inducer chromosome used. The I−R-induced lethals occur along the entire length of the X chromosome. These sites probably correspond to specific loci which are more or less homologous with I. The complementation relationshups showed that some specific loci were more frequently involved in all the lethal mutations tested. The most sensitive loci are, in order of observation: l(1)J1, ct, f, ma¹ and m. Among induced recessive lethals considered to be point mutation, complementation tests showed that many of them are in fact multilocius deficiencies which can be detected only at the molecular level.

It seems that the production of I−R rearrangements (cytologically visible or not) may be the most important mechanism leading to lethal mutations. These mutations probably occur during the transposition of I elements, hence their importance from an evolutionary standpoint.     

THE ROLE OF GENES OF LARGE EFFECT ON INBREEDING DEPRESSION IN MIMULUS GUTTATUS

Severe inbreeding depression is routinely observed in outcrossing species. If inbreeding load is due largely to deleterious alleles of large effect, such as recessive lethals or steriles, then most of it is expected to be purged during brief periods of inbreeding. In contrast, if inbreeding depression is due to the cumulative effects of many deleterious alleles of small effect, then it will be maintained in the face of periodic inbreeding. Whether or not inbreeding depression can be purged with inbreeding in the short term has important implications for the evolution of mating systems and the probability that a small population will go extinct. In this paper I evaluate the extent to which the tremendous inbreeding load in a primarily outcrossing population of the wildflower, Mimulus guttatus, is due to alleles of large effect. To do this, I first constructed a large outbred “ancestral” population by randomly mating plants collected as seeds from a natural population. From this population I formed 1200 lines that were maintained by self-fertilization and single seedling descent: after five generations of selling, 335 lines had survived the inbreeding process. Selection during the line formation is expected to have largely purged alleles of large effect from the collection of highly inbred lines. Because alleles with minor effects on fitness should have been effectively neutral, the inbreeding depression due to this class of genes should have been unchanged. The inbred lines were intercrossed to form a large, outcrossed “purged” population. Finally, I estimated the fitness of outbred and selfed progeny from the ancestral and purged populations to determine the contribution of major deleterious alleles on inbreeding depression. I found that although the average fitness of the outcrossed progeny nearly doubled following purging, the limited decline in inbreeding depression and limited increase in inbred fitness indicates that alleles of large effect are not the principle cause of inbreeding depression in this population. In aggregate, the data suggest that lethals and steriles make a minority contribution to inbreeding depression and that the increased outbred fitness is due primarily to adaptation to greenhouse conditions.     

Plasticity of plant defense and its evolutionary implications in wild populations of Boechera stricta

Phenotypic plasticity is thought to impact evolutionary trajectories by shifting trait values in a direction that is either favored by natural selection (“adaptive” plasticity) or disfavored (“nonadaptive” plasticity). However, it is unclear how commonly each of these types of plasticity occurs in natural populations. To answer this question, we measured glucosinolate defensive chemistry and reproductive fitness in over 1500 individuals of the wild perennial mustard Boechera stricta, planted in four common gardens across central Idaho, United States. Glucosinolate profiles—including total glucosinolate concentration as well as the relative abundances and overall diversity of different compounds—were strongly plastic both among habitats and within habitats. Patterns of glucosinolate plasticity varied greatly among genotypes. Plasticity among sites was predicted to affect fitness in 27.1% of cases; more often than expected by chance, glucosinolate plasticity increased rather than decreased relative fitness. In contrast, we found no evidence for within‐habitat selection on glucosinolate reaction norm slopes (i.e., plasticity along a continuous environmental gradient). Together, our results indicate that glucosinolate plasticity may improve the ability of B. stricta populations to persist after migration to new habitats.     

Chromosomal Effects on Mutability in the P-M System of Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

Two manifestations of hybrid dysgenesis were studied in flies with chromosomes derived from two different P strains. In one set of experiments, the occurrence of recessive X-linked lethal mutations in the germ cells of dysgenic males was monitored. In the other, the behavior of an X-linked P-element insertion mutation, snw, was studied in dysgenic males and also in dysgenic females. The chromosomes of one P strain were more proficient at causing dysgenesis in both sets of experiments. However, there was variation among the chromosomes of each strain in regard to the ability to induce lethals or to destabilize snw. The X chromosome, especially when it came from the stronger P strain, had a pronounced effect on both measures of dysgenesis, but in combination with the major autosomes, these effects were reduced. For the stronger P strain, the autosomes by themselves contributed significantly to the production of X-linked lethals and also had large effects on the behavior of snw, but they did not act additively on these two characters. For this strain, the effects of the autosomes on the X-linked lethal mutation rate suggest that only 1/100 P element transpositions causes a recessive lethal mutation. For the weaker P strain, the autosomes had only slight effects on the behavior of snw and appeared to have negligible effects on the X-linked lethal mutation rate. Combinations of chromosomes from either the strong or the weak P strain affected both aspects of dysgenesis in a nonadditive fashion, suggesting that the P elements on these chromosomes competed with each other for transposase, the P-encoded function that triggers P element activity. Age and sex also influenced the ability of chromosomes and combinations of chromosomes to cause dysgenesis.     

Heterozygous effects on viability of Drosophila supergenes that are lethal when homozygous

Eight fourth chromosomes which were homozygous lethal and 170 which were homozygous nonlethal were extracted from the same Drosophila melanogaster cage. The lethals were complementary, i.e., they were viable in all 28 nonreflexive pairwise combinations. Three different lethals produced sterile homozygotes; these are called leaky lethals. Different lethal heterozygotes' viabilities were compared by means of paired-t tests. The difference in mean relative viabilities between a pair of genotypes containing different lethals but exactly the same nonlethal was treated as one observation. The mean difference for any pair of lethals was based on only part of the full array of nonlethals. Of 17 possible paired comparisons, nine were statistically significant. In eight out of ten possible pairs and in six out of seven significant pairs, the heterozygous viability of leaky lethals was less than that of absolute (nonleaky) lethals. There was no association between stage of homozygous lethal action and heterozygous viability effect. In general, different lethals had different heterozygous effects on viability. The results are summarized in Table 5. In memoriam David Walter Kenyon (1939–1972)Research supported by The National Science Foundation of the United States (GB-3759).     

OPTIMAL OUTCROSSING IN IPOMOPSIS AGGREGAT A: SEED SET AND OFFSPRING FITNEs

Restricted gene flow and localized selection should establish a correlation between physical proximity and genetic similarity in many plant populations. Given this situation, fitness may decline in crosses between nearby plants (inbreeding depression), and in crosses between more widely separated plants (“outbreeding depression”) mostly as a result of disruption of local adaptation. It follows that seed set and offspring fitness may be greatest in crosses over an intermediate “optimal outcrossing distance.” This prediction was supported for Ipomopsis aggregata, a long-lived herbaceous plant pollinated by hummingbirds. In six replicate pollination experiments, mean seed set per flower was higher with an outcrossing distance of 1–10 m than with selfing or outcrossing over 100 m. A similar pattern appeared in the performance of offspring from experimental crosses grown under natural conditions and censused for a seven-year period. Offspring from 10-m crosses had higher survival, greater chance of flowering, and earlier flowering than those from 1-m or 100-m crosses. As a result, 1-m and 100-m offspring achieved only 47% and 68%, respectively, of the lifetime fitness of 10-m offspring. Offspring fitness also declined with planting distance from the seed parent over a range of 1–30 m, so that adaptation to the maternal environment is a plausible mechanism for outbreeding depression. Censuses in a representative I. aggregata population indicated that the herbaceous vegetation changes over a range of 2–150 m, suggesting that there is spatial variation in selection regimes on a scale commensurate with the observed effects of outbreeding depression and planting distance. We discuss the possibility that differences in seed set might in part reflect maternal mate discrimination and emphasize the desirability of measuring offspring fitness under natural conditions in assessing outcrossing effects.     

The effects of radiation exposure-rate and exposure fractionation on the frequencies of recessive and dominant lethals in immature oocytes of Drosophila melanogaster

A measure of seedling viability was used to estimate the homozygous geneticc load of seedlings from mutagenized populations of Oenothera. The breeding protocol forced genomes to homozygosity. Pollen from control and mutagenized Oenothera hookeri T. and G. strain Johansen, a seven-paired lethal-free stock, was used to pollinate a translocation stock with balanced lethals. The hybrid formed a complete translocation ring and upon selfing yielded two types of plants, a translocation heterozygote similar to its hybrid parent and a seven-paired plant homozygous for the 7 chromosomes obtained from Johansen. Genetic markers allowed the identification of seven-paired seedlings in the cotyledon stage. Control hybrids averaged a recovery of 57.5% seven-paired seedlings. Hybrids obtained from plants that had been mutagenized by seed treatment with 15000 R X-rays, 0.04 M ethyl methanesulfonate (EMS), and 0.08 M EMS averaged 48.3%, 19.2%, and 6.0% recovery of seven-paired forms, respectively. The data are used to estimate the genetic load and lethal equivalents in each population. The implications of these results are evaluated with reference to mutation breeding of plant populations.     

Patterns and dynamics of rapid local adaptation and sex in varying habitat types in rotifers

Local adaptation is an important principle in a world of environmental change and might be critical for species persistence. We tested the hypothesis that replicated populations can attain rapid local adaptation under two varying laboratory environments. Clonal subpopulations of the cyclically parthenogenetic rotifer Brachionus calyciflorus were allowed to adapt to two varying harsh and a benign environment: a high‐salt, a food‐limited environment and untreated culture medium (no salt addition, high food). In contrast to most previous studies, we re‐adjusted rotifer density to a fixed value (two individuals per ml) every 3–4 days of unrestricted population growth, instead of exchanging a fixed proportion of the culture medium. Thus our dilution regime specifically selected for high population growth during the entire experiment and it allowed us to continuously track changes in fitness (i.e., maximum population growth under the prevailing conditions) in each population. After 56 days (43 asexual and eight sexual generations) of selection, the populations in the harsh environments showed a significant increase in fitness over time relative to the beginning compared to the population in untreated culture medium. Furthermore, the high‐salt population exhibited a significantly elevated ratio of sexual offspring from the start of the experiment, which suggested that this environment either triggered higher rates of sex or that the untreated medium and the food‐limited environment suppressed sex. In a following assay of local adaptation we measured population fitness under “local” versus “foreign” conditions (populations adapted to this environment compared to those of the other environment) for both harsh habitats. We found significantly higher fitness values for the local populations (on average, a 38% higher fitness) compared to the foreign populations. Overall, local adaptation was formed rapidly and it seemed to be more pronounced in the high‐salt treatment.     

Dynamics of genetic rescue in inbred Drosophila melanogaster populations

Genetic rescue has been proposed as a management strategy to improve the fitness of genetically eroded populations by alleviating inbreeding depression. We studied the dynamics of genetic rescue in inbred populations of Drosophila. Using balancer chromosomes, we show that the force of heterosis that accompanies genetic rescue is large and allows even a recessive lethal to increase substantially in frequency in the rescued populations, particularly at stress temperatures. This indicates that deleterious alleles present in the immigrants can increase significantly in frequency in the recipient population when they are in linkage disequilibrium with genes responsible for the heterosis. In a second experiment we rescued eight inbred Drosophila populations with immigrants from two other inbred populations and observe: (i) there is a significant increase in viability both 5 and 10 generations after the rescue event, showing that the increase in fitness is not transient but persists long-term. (ii) The lower the fitness of the recipient population the larger the fitness increase. (iii) The increase in fitness depends significantly on the origin of the rescuers. The immigrants used were fixed for a conditional lethal that was mildly deleterious at 25°C but lethal at 29°C. By comparing fitness at 25°C (the temperature during the rescue experiment) and 29°C, we show that the lethal allele reached significant frequencies in most rescued populations, which upon renewed inbreeding became fixed in part of the inbred lines. In conclusion, in addition to the fitness increase genetic rescue can easily result in a substantial increase in the frequency of mildly deleterious alleles carried by the immigrants. This can endanger the rescued population greatly when it undergoes recurrent inbreeding. However, using a sufficient number of immigrants and to accompany the rescue event with the right demographic measures will overcome this problem. As such, genetic rescue still is a viable option to manage genetically eroded populations.     

Darwinian fitness and adaptedness in experimental populations of Drosophila willistoni

Fifteen second chromosomes were extracted from Drosophila willistoni flies collected in four natural populations. The adaptedness of populations homozygous for each chromosome was measured by average population size and productivity. Six control populations were established with mixtures of the wild second chromosomes. The Darwinian fitness of flies homozygous for each wild second ehromosome, and of flies carrying random combinations of these chromosomes, was measured relative to the fitness of flies heterozygous for a wild and a marker chromosome. The Darwinian fitness of homozygotes for each second chromosome relative to the fitness of flies carrying random combinations of the natural chromosomes was then inferred. The estimated loss of fitness on making the natural second chromosomes homozygous was substantial, ranging from 39 to 83 pereent, with an average reduction in fitness of 66 percent. These results with D. willistoni are consistent with those from similar experiments with other drosophila species, and they are compatible with a significant role for heterosis in the maintenance of genetic variability.Populations homozygous for wild chromosomes differ in their adaptedness to the experimental environment. Population size and productivity are correlated, although the correlation is far from complete. Some populations have high productivity and low population size, or vice versa. The control populations, with greater genetic variability, were superior in adaptedness to the average of the single-chromosome populations. The Darwinian fitness and the adaptedness of the genotypes in this experiment were not significantly correlated. It follows that certain measures used by population geneticists, such as genetic load and average Darwinian fitness, cannot be taken as general indices of how well adapted a population is to its environment.This work was supported by U.S. Public Health Service Grant RO1-HDO5055, NSF grant GB-20694 (International Biological Program). AEC contract AT-(30-1) 3096, and PHS Career Development Award K3 GM 37265. The collection of the flies was supported by the Fundacão de Amparo a Pesquisa do Estado de São Paulo, Brazil. The senior author's stay in New York, where the experiments were conducted, was financed in part by Research Fellowship 2-12861 from the Panamerican Union.     

GENE‐DOSAGE EFFECTS ON FITNESS IN RECENT ADAPTIVE DUPLICATIONS: ace‐1 IN THE MOSQUITO CULEX PIPIENS

Gene duplications have long been advocated to contribute to the evolution of new functions. The role of selection in their early spread is more controversial. Unless duplications are favored for a direct benefit of increased expression, they are likely detrimental. In this article, we investigated the case of duplications favored because they combine already functionally divergent alleles. Their gene‐dosage/fitness relations are poorly known because selection may operate on both overall expression and duplicates relative dosage. Using the well‐documented case of Culex pipiens resistance to insecticides, we compared strains with various ace‐1 allele combinations, including two duplicated alleles carrying both susceptible and resistant copies. The overall protein activity was nearly additive, but, surprisingly, fitness correlated better with the relative proportion of susceptible and resistant copies rather than any absolute measure of activity. Gene dosage is thus crucial, duplications stabilizing a “heterozygote” phenotype. It corroborates the view that these were favored because they fix a permanent heterosis, thereby solving the irreducible trade‐off between resistance and synaptic transmission. Moreover, we showed that the contrasted successes of the two duplicated alleles in natural populations depend on genetic changes unrelated to ace‐1, confirming the probable implication of recessive sublethal mutations linked to structural rearrangements in some duplications.     

INTERSPECIFIC INCOMPATIBILITY IN GOSSYPIUM. II. LIGHT AND ELECTRON MICROSCOPE STUDIES OF CELL NECROSIS AND TUMORIGENESIS IN HYBRIDS OF G. KLOTZSCHIANUM

Many Gossypium interspecific hybrids that involve G. klotzschianum result in either embryo or seedling lethals. Histologically, lethal symptoms are characterized by necrotic cells and tumors that appear 10–15 days after fertilization in the embryo lethals and at the first true-leaf stage in the seedling lethals. Ultrastructural studies of cell necrosis in seedling lethals show that the first subcellular abnormality is the degeneration of the inner membrane and cristae of the mitochondria. Mitochondrial degeneration is essentially complete before other organelles show evidence of structural aberrations.