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Sex-Determining Mechanisms Help (page 3)

By — McGraw-Hill Professional
Updated on Apr 25, 2014

Haplodiploidy

Male bees are known to develop parthenogenetically (without union of gametes) from unfertilized eggs (arrhenotoky) and are therefore haploid. Females (both workers and queens) originate from fertilized (diploid) eggs. Sex chromosomes are not involved in this mechanism of sex determination, which is characteristic of the insect order Hymenoptera including the ants, bees, wasps, etc. The quantity and quality of food available to the diploid larva determines whether that female will become a sterile worker or a fertile queen. Most of the eggs laid in the hive will be fertilized and develop into worker females. Those eggs that the queen does not fertilize from her store of sperm in the seminal receptacle will develop into haploid males.

Single-Gene Effects

Complementary Sex Determination (CSD) Factors

In addition to haplodiploidy, members of the insect order Hymenoptera are known to produce males by homozygosity at a single-gene locus. This has been confirmed in the tiny parasitic wasp Bracon hebetor (often called Habrobracon juglandis), as well as in bees. At least nine sex alleles are known at this locus in Bracon and may be represented by sa, sb, sc, … , si. All females must be heterozygotes such as sasb, sasc, sdsf, etc. If an individual is homozygous for any of these alleles such as sasa, scsc, etc., it develops into a diploid male (usually sterile). Haploid males, of course, would carry only one of the alleles at this locus, e.g., sa, sc, or sg, etc.

Single-Gene Effects

"Mating Type" in Microorganisms

In microorganisms such as the alga Chlamydomonas, the fungi Neurospora, and yeast, sex is under the control of a single gene. Haploid individuals possessing the same allele of this "mating-type" locus usually cannot fuse with each other to forma zygote, but haploid cells containing opposite (complementary) alleles at this locus may fuse. Asexual reproduction in the single-celled motile alga Chlamydomonas reinhardi usually involves two mitotic divisions within the old cell wall (Fig. 5-1). Rupture of the sporangium releases the new generation of haploid zoospores. If nutritional requirements are satisfied, asexual reproduction may go on indefinitely. In unfavorable conditions where nitrogen balance is upset, daughter cells may be changed to gametes. Genetically, there are two mating types, plus (+) and minus (–), which are morphologically indistinguishable and therefore called isogametes. Fusion of gametes unites two entire cells into a diploid nonmotile zygote that is relatively resistant to unfavorable growth conditions. With the return of conditions that favor growth, the zygote undergoes meiosis and forms four motile haploid daughter cells (zoospores), two of plus and two of minus mating type.

Single-Gene Effects

Practice problems for these concepts can be found at:

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