Question

In a number of organisms, including Drosophila and butterflies, genes that alter the sex ratio have been described. In the pest species Musca domesticus (the house fly), Aedes aegypti (the mosquito that is the vector for yellow fever), and Culex pipiens (the mosquito vector for filariasis and some viral dis- eases), scientists are especially interested in such genes. Sex in Culex is determined by a single gene pair, \(M m\) being male and \(m m\) being female. Males homozygous for the recessive gene \(d d\) never produce many female offspring. The \(d d\) combination in males causes fragmentation of the \(m\) -bearing dyad during the first meiotic division, hence its failure to complete spermatogenesis. (a) Account for this sex-ratio distortion by drawing labeled chromosome arrangements in primary and secondary spermatocytes for each of the following genotypes: \(M m D d\) and \(M m d d .\) How do meiotic products differ between \(D d\) and \(d d\) genotypes? Note that the diploid chromosome number is 6 in Culex pipiens and both \(D\) and \(M\) loci are linked on the same chromosome. (b) How might a sex-ratio distorter such as \(d d\) be used to control pest population numbers?

Short answer

Question: Explain the sex-ratio distortion in house flies and mosquitoes with genotypes \(MmDd\) and \(Mmdd\), and how this can be used to control pest population numbers. Answer: In the \(MmDd\) genotype, four viable spermatocytes are produced during spermatogenesis: \(MD\), \(mD\), \(Md\), and \(md\). However, in the \(Mmdd\) genotype, only three viable spermatocytes are produced: \(MD\), \(Md\), and \(md\), with the \(m\)-bearing dyad undergoing fragmentation and not completing spermatogenesis. The lack of viable sperm containing \(m\) in the \(Mmdd\) genotype results in a skewed sex ratio, producing more male offspring. This sex-ratio distortion can be used to control pest populations by releasing genetically-engineered males carrying the \(dd\) genotype into the wild. These males will mate with wild females, and since the \(dd\) genotype produces fewer fertile female offspring, the pest population will decrease due to the reduced number of breeding females. Over time, this method could significantly reduce the number of pests in the population.

Step-by-step solution

Chromosome Arrangements for MmDd Genotype

For the genotype \(MmDd\), we organize the chromosome arrangements in primary and secondary spermatocytes: Primary Spermatocytes: \(M_mD_DD\), \(M_mD_dd\), \(MmDd\) Secondary Spermatocytes: \(MD\), \(mD\), \(Md\), \(md\) Now let's move to the \(Mmdd\) genotype.

Chromosome Arrangements for Mmdd Genotype

For the genotype \(Mmdd\), we organize the chromosome arrangements in primary and secondary spermatocytes: Primary Spermatocytes: \(M_mD_dd\) Secondary Spermatocytes: Since \(dd\) in males causes fragmentation of the \(m\)-bearing dyad during the first meiotic division, one spermatocyte (containing \(m\)) will fail to complete spermatogenesis, leading to only three viable spermatocytes: \(MD\), \(Md\), \(md\)

Compare Meiotic Products in Dd and dd Genotypes

In the \(Dd\) genotypes, four viable spermatocytes are produced during spermatogenesis: \(MD\), \(mD\), \(Md\), and \(md\). However, in the \(dd\) genotypes, only three viable spermatocytes are produced: \(MD\), \(Md\), and \(md\). The viable sperm containing \(m\) will not be produced in \(dd\) genotypes, causing a skewed sex ratio with more male offspring.

Discuss the Use of dd to Control Pest Population Numbers

A sex-ratio distorter such as \(dd\) can be used to control pest population numbers by releasing genetically-engineered males carrying the \(dd\) genotype into the wild. These males will mate with wild females, and since the \(dd\) genotype influences the sex ratio by producing fewer fertile female offspring, the pest population will decrease due to fewer breeding females. Over time, this method could significantly reduce the number of pests.

Key concepts

Chromosome Arrangements

Chromosome arrangements play a crucial role in determining genetic outcomes. In the house fly and mosquito species like Culex pipiens, specific genes on chromosomes decide the sex of the offspring. In organisms like Culex, sex is determined by a pair of genes, with the male being identified as \(Mm\) and the female as \(mm\). Understanding the arrangements involves organizing the chromosomes of genotypes such as \(MmDd\) or \(Mmdd\) in both primary and secondary spermatocytes.

Consider the genotype \(MmDd\): during meiotic divisions, the chromosomes organize as follows: primary spermatocytes have arrangements like \(M_mD_DD\), resulting in secondary spermatocytes of \(MD\), \(mD\), \(Md\), and \(md\). For \(Mmdd\), though still starting with \(M_mD_dd\) in primary spermatocytes, a malfunction caused by the homozygous \(dd\) gene results in a loss of the \(m\)-bearing dyad. Consequently, secondary spermatocytes end up with \(MD\), \(Md\), and \(md\) only. The failure to produce an \(m\)-bearing sperm in \(dd\) genotypes skews the sex ratio heavily towards male production. This highlights the intricate interplay between chromosome arrangements and genetic outcomes.

Meiotic Division

Meiotic division is a fundamental biological process necessary for sexual reproduction, ensuring the formation of gametes. Through meiosis, organisms reduce their chromosomal number by half, forming sperm and egg cells. In pest species like Culex pipiens, meiotic division determines the sex of the offspring, influenced by specific gene combinations. During meiosis, the typical cell undergoes two divisions: meiosis I (reductional division) and meiosis II (equational division).

In genotypes such as \(Dd\) and \(dd\), this process plays a pivotal role. For \(Dd\), a typical meiotic division results in four viable sperm cells embodying different combinations of the \(D\), \(M\), and \(m\) alleles: \(MD\), \(mD\), \(Md\), and \(md\). However, for \(dd\) genotypes, a critical deviation occurs due to the fragmentation of the \(m\)-bearing dyad, leading to only three viable sperm cells: \(MD\), \(Md\), and \(md\). This problem is notable in the first meiotic division and manifests in the reduced number of fertile offspring, primarily producing male progeny due to the lack of viable \(m\)-bearing sperm. These meiotic divisions are pivotal in skewing sex ratios and can be leveraged for pest control strategies.

Pest Population Control

Pest population control is essential for maintaining ecological balance and preventing the spread of diseases carried by pests such as mosquitoes. Harnessing genetic tools can provide innovative solutions, such as the introduction of sex-ratio distortion mechanisms.

In regard to the \(dd\) homozygous genotype in pests like mosquitoes, these males can be utilized to strategically reduce pest populations. By releasing engineered males carrying the \(dd\) genotype into the wild, these males will mate but produce fewer female offspring due to the disrupted meiotic process that affects the sex ratio. This results in an overall lowering of fertile female numbers capable of laying eggs, hence reducing the pest population.

The success of using such genetic techniques lies in its targeted approach: by aligning natural breeding behaviors with scientific intervention, populations dwindle over successive generations. Although further research is needed to mitigate ecological impacts, deploying sex-ratio distortion in pest management remains a promising area of study.