Genetic variation and asexual reproduction in the facultatively parthenogenetic cockroach Nauphoeta cinerea: implications for the evolution of sex.

Asexual reproduction could offer up to a two-fold fitness advantage over sexual reproduction, yet higher organisms usually reproduce sexually. Even in facultatively parthenogenetic species, where both sexual and asexual reproduction is sometimes possible, asexual reproduction is rare. Thus, the debate over the evolution of sex has focused on ecological and mutation-elimination advantages of sex. An alternative explanation for the predominance of sex is that it is difficult for an organism to accomplish asexual reproduction once sexual reproduction has evolved. Difficulty in returning to asexuality could reflect developmental or genetic constraints. Here, we investigate the role of genetic factors in limiting asexual reproduction in Nauphoeta cinerea, an African cockroach with facultative parthenogenesis that nearly always reproduces sexually. We show that when N. cinerea females do reproduce asexually, offspring are genetically identical to their mothers. However, asexual reproduction is limited to a nonrandom subset of the genotypes in the population. Only females that have a high level of heterozygosity are capable of parthenogenetic reproduction and there is a strong familial influence on the ability to reproduce parthenogenetically. Although the mechanism by which genetic variation facilitates asexual reproduction is unknown, we suggest that heterosis may facilitate the switch from producing haploid meiotic eggs to diploid, essentially mitotic, eggs.

Developmental constraints on the mode of reproduction in the facultatively parthenogenetic cockroach Nauphoeta cinerea.

Considerable work in evolutionary biology has focused on the question of why sex persists. Both advantages to sex and constraints limiting a return to asexual reproduction are hypothesized to maintain sex once it evolves. Developmental constraints would limit asexual reproduction from a sexual species if it were difficult for females to switch from making eggs that do not develop without fertilization to making zygotes that are capable of developing in the absence of fertilization. Nauphoeta cinerea is an ovoviviparous cockroach in which some females are capable of switching from a sexual mode of reproduction to an asexual mode when isolated from males. Yet, while facultative parthenogenesis can occur in individuals, few females make the switch. Thus, this cockroach provides an ideal system for examining the potential role of developmental constraints in maintaining sex. Here we compare the cytogenetics and embryonic development of sexual and parthenogenetic offspring in N. cinerea. We find that deviations from normal ploidy levels are associated with abnormal development. All viable N. cinerea embryos exhibit typically hemimetabolous insect embryogenesis. Although there is no variation among embryos in development within a sexually produced clutch, we see extreme variation in asexually derived clutches. These results suggest that developmental constraints limit the success of asexual reproduction in this facultatively parthenogenetic cockroach. Our data further suggest that the specific constraint occurs in the switch from a meiotic mode of reproduction requiring fertilization to diploid zygotes that develop in the absence of fertilization.

Dental treatment and late prosthetic joint infections.

Hospital and dental charts of 2,693 patients in whom total prosthetic joints had been placed at the Veterans Administration Hospitals of Ann Arbor and Allen Park, Michigan, as well as at The University of Michigan Hospital, were analyzed. Of the thirty (1.1%) late prosthetic joint infections (greater than 6 months after placement), only one (0.04%) could be temporally associated with dental treatment. A Fisher's exact test of the data reflected that dental treatment in this population did not increase the incidence of late prosthetic joint infections (p value is 0.0005). Nine of the thirty late infections occurred in insulin-dependent diabetic patients and patients on long-term immunosuppressive therapy. An analysis of the organisms isolated from the late infections shows that 54% where Staphylococcus epidermidis and Staphylococcus aureus. These data do not support the practice of prescribing prophylactic antibiotic coverage of prosthetic hip and knee joints prior to all dental therapy. Rather, use of antibiotics during dental treatment appears warranted only if a chronic bacteremia is anticipated or where a predisposing systemic condition may exist.

Increased occlusal vertical dimension in adult monkeys.

Splints which opened the vertical dimensions on all posterior teeth in five rhesus monkeys were studied over a period of 3 to 36 months. Findings were as follows: 1. The increased vertical dimension of occlusion did not have a pathologic effect on the TMJs. 2. The posterior teeth attached to the splint and the teeth opposing the splint were intruded into the alveolar processes. 3. The nonoccluding incisor teeth extruded, but the epithelial attachment remained close to the cementoenamel junction. 4. The vertical dimension as measured between the markers in the bones of the mandible and maxillae gradually returned toward the pretreatment vertical dimension, although complete closure to this position was not accomplished during the time period of the experiment. 5. The relationship between the epithelial attachment and the cementoenamel junction remained undisturbed by the experiment. 6. After completion of growth, there appeared to be a definite tendency for rebound to the normal pretreatment vertical dimension following increase of the vertical dimension. This was recorded by measurable movements of the teeth.

Effects of electrosurgery on the gingival attachment in rhesus monkeys.

A biometric and histometric study of electrosurgical gingival "troughing" with a fully rectified current was accomplished in four Rhesus monkeys. The operation resulted in a statistically significant recession of the free gingival margin and loss of connective tissue attachment associated with apical migration of the junctional epithelium. Burn marks from contacts with the electrodes were observed on the cemental surfaces and in the dentin under the enamel close to the cemento-enamel junction. The cemental burn marks usually were covered by epithelium, which may explain the apical migration of the junctional epithelium observed in the experiemtal teeth. A slight loss of crestal alveolar bone occurred in the experimental areas and a bone sequestrum formed in one instance. Secondary dentin was found in response to electrosurgical contact of the cemental surface.

Lateral displacement of the mandible in Rhesus monkeys.

Lateral displacement of the mandible was induced experimentally in five adult male Rhesus monkeys. Following from 2 1/2 weeks to 19 months, the animals were sacrificed and examined histologically. Two of the monkeys received H3 proline 4 months prior to sacrifice and radioautographs were prepared. The lateral displacement apparently caused great discomfort because the monkeys refused to eat and lost weight rapidly for a few months. After about 12 months, the monkeys could again eat their normal diet to the extent they started to gain weight slowly. The histological and radio-autographic findings indicated almost no changes of the articular surfaces of the temporomandibular joints. However, extensive reorganization of the neck of the condyles was observed. Severe periodontal trauma and extensive movements of the teeth occurred in spite of the fact that the teeth in each jaw were splinted firmly together.

The storage of teeth before reimplantation in monkeys. A histologic study.

This study is a histologic comparison of twenty teeth which were extracted, endodontically treated, stored for 7 days at +4 degrees C. and at -10 degrees C., and subsequently reimplanted in ten monkeys. In these monkeys, as controls, ten more teeth were extracted, endodontically treated, and immediately reimplanted. Up to 1 year, 100 per cent success was obtained in those teeth stored at +4 degrees C. before reimplantation, and only 50 per cent success was obtained in those teeth stored at -10 degrees C. before reimplantation.