Genetic and reproductive consequences of consanguineous marriage in Bangladesh

Introduction This study aimed to assess the prevalence, sociodemographic factors, reproductive consequences, and heritable disease burdens associated with consanguineous marriage (CM) in Bangladesh. Methods A total of 7,312 families, including 3,694 CM-families, were recruited from 102 locales of 58 districts of Bangladesh. Using a standard questionnaire, we collected medical history and background sociodemographic data of these families. Family history was assessed by pedigree analysis. Fertility, mortality, secondary sex ratio, selection intensity, lethal equivalents were measured using standard methods. Results The mean prevalence of CM in our studied population was 6.64%. Gross fertility was higher among CM families, as compared to the non-CM families (p < 0.05). The rate of under-5 child (U5) mortality was significantly higher among CM families (16.6%) in comparison with the non-CM families (5.8%) (p < 0.01). We observed a persuasive rise of abortion/miscarriage and U5 mortality rates with the increasing level of inbreeding. The value of lethal equivalents per gamete found elevated for autosomal inheritances as compared to sex-linked inheritance. CM was associated with the incidence of several single-gene and multifactorial diseases, and congenital malformations, including bronchial asthma, hearing defect, heart diseases, sickle cell anemia (p < 0.05). The general attitude and perception toward CM were rather indifferent, and very few people were concerned about its genetic burden. Conclusion This study highlights the harmful consequences of CM on reproductive behavior and the incidence of hereditary conditions. It essences the need for genetic counseling from premarital to postnatal levels in Bangladesh.

Genetic Allee effects and their interaction with ecological Allee effects

SummaryIt is now widely accepted that genetic processes such as inbreeding depression and loss of genetic variation can increase the extinction risk of small populations. However, it is generally unclear whether extinction risk from genetic causes gradually increases with decreasing population size or whether there is a sharp transition around a specific threshold population size. In the ecological literature, such threshold phenomena are called “strong Allee effects” and they can arise for example from mate limitation in small populations.In this study, we aim to a) develop a meaningful notion of a “strong genetic Allee effect”, b) explore whether and under what conditions such an effect can arise from inbreeding depression due to recessive deleterious mutations, and c) quantify the interaction of potential genetic Allee effects with the well-known mate-finding Allee effect.We define a strong genetic Allee effect as a genetic process that causes a population’s survival probability to be a sigmoid function of its initial size. The inflection point of this function defines the critical population size. To characterize survival-probability curves, we develop and analyze simple stochastic models for the ecology and genetics of small populations.Our results indicate that inbreeding depression can indeed cause a strong genetic Allee effect, but only if individuals carry sufficiently many deleterious mutations (lethal equivalents) on average and if these mutations are spread across sufficiently many loci. Populations suffering from a genetic Allee effect often first grow, then decline as inbreeding depression sets in, and then potentially recover as deleterious mutations are purged. Critical population sizes of ecological and genetic Allee effects appear to be often additive, but even superadditive interactions are possible.Many published estimates for the number of lethal equivalents in birds and mammals fall in the parameter range where strong genetic Allee effects are expected. Unfortunately, extinction risk due to genetic Allee effects can easily be underestimated as populations with genetic problems often grow initially, but then crash later. Also interactions between ecological and genetic Allee effects can be strong and should not be neglected when assessing the viability of endangered or introduced populations.

Yield Variation among Clones of Lowbush Blueberry as a Function of Genetic Similarity and Self-compatibility

Two types of field hand crosses (pairwise touching-neighbor and a full 5 × 5 diallel, Griffing's Model 2, Method 3) were performed in combination with genetic similarity estimations of mating partners using expressed sequence tag–polymerase chain reaction molecular markers to elucidate genetic factors underlying yield variations among clones (genotypes) of lowbush blueberry (Vaccinium angustifolium) in two managed fields in Maine. Genetic similarity values for touching pairs ranged from 0.308 to 0.765. Based on pairwise touching-neighbor crosses, no evidence was found for yield being affected by genetic similarity. However, self-fertility of clones was a significant positive predictor of outcross yields. The calculation of lethal equivalents, derived from selfing to outcross ratios, showed a large range in genetic load among clones and a higher average load than that previously reported in the related highbush blueberry (V. corymbosum). The diallel experiment revealed significant general and specific combining ability for all three post-pollination yield traits measured (proportion fruit set, mean mature seed per pollination, and mean berry weight per pollination). Narrow-sense heritability estimates for all three yield traits were moderately high (h2 = 0.58, 0.46, and 0.56, respectively). It is concluded that phenotypically screening for self-compatible clone yield attributes could be useful in identifying germplasm candidates for breeding and propagation.

Whole-Genome Characterization of Embryonic Stage Inbreeding Depression in a Selfed Loblolly Pine Family

Inbreeding depression is important in the evolution of plant populations and mating systems. Previous studies have suggested that early-acting inbreeding depression in plants is primarily due to lethal alleles and possibly epistatic interactions. Recent advances in molecular markers now make genetic mapping a powerful tool to study the genetic architecture of inbreeding depression. We describe a genome-wide evaluation of embryonic viability loci in a selfed family of loblolly pine (Pinus taeda L.), using data from AFLP markers from an essentially complete genome map. Locus positions and effects were estimated from segregation ratios using a maximum-likelihood interval mapping procedure. We identified 19 loci showing moderately deleterious to lethal embryonic effects. These loci account for &gt;13 lethal equivalents, greater than the average of 8.5 lethal equivalents reported for loblolly pine. Viability alleles show predominantly recessive action, although potential overdominance occurs at 3 loci. We found no evidence for epistasis in the distribution of pairwise marker correlations or in the regression of fitness on the number of markers linked to deleterious alleles. The predominant role of semilethal alleles in embryonic inbreeding depression has implications for the evolution of isolated populations and for genetic conservation and breeding programs in conifers.

Dynamics of inbreeding depression due to deleterious mutations in small populations: mutation parameters and inbreeding rate

A multilocus stochastic model is developed to simulate the dynamics of mutational load in small populations of various sizes. Old mutations sampled from a large ancestral population at mutation–selection balance and new mutations arising each generation are considered jointly, using biologically plausible lethal and deleterious mutation parameters. The results show that inbreeding depression and the number of lethal equivalents due to partially recessive mutations can be partly purged from the population by inbreeding, and that this purging mainly involves lethals or detrimentals of large effect. However, fitness decreases continuously with inbreeding, due to increased fixation and homozygosity of mildly deleterious mutants, resulting in extinctions of very small populations with low reproductive rates. No optimum inbreeding rate or population size exists for purging with respect to fitness (viability) changes, but there is an optimum inbreeding rate at a given final level of inbreeding for reducing inbreeding depression or the number of lethal equivalents. The interaction between selection against partially recessive mutations and genetic drift in small populations also influences the rate of decay of neutral variation. Weak selection against mutants relative to genetic drift results in apparent overdominance and thus an increase in effective size (Ne) at neutral loci, and strong selection relative to drift leads to a decrease in Ne due to the increased variance in family size. The simulation results and their implications are discussed in the context of biological conservation and tests for purging.

Effects of Inbreeding on Reproductive Losses in Kota Tribe

Sanghvi's hypothesis on long term effects of inbreeding was tested in Kotas. Kota is a numerically small tribal population in the Nilgiri district, Tamil Nadu State, India. Consanguineous marriages are common in this tribe. A total of 95 couples were taken for this study and necessary data were collected on a set proforma. Of the 95 couples, 28 (29.5%) were consanguineously related. The inbreeding coefficient for autosomal genes is 0.022 and for sex-linked genes is 0.03. Inbreeding effects on reproductive losses were examined through an exponential regression model. Although the regression coefficient B values are positive, they are insignificant, suggesting no consistent relationship between degree of consanguinity and the reproductive losses. The estimates of genetic load is 1.8 lethal equivalents per gamete and the average B/A ratio is 5. These findings empirically support the Sanghvi's contention.