Parthenogenesis

The ability of unfertilized eggs to develop is common to many insect species and in some is the normal mode of reproduction under certain conditions. In all insects except Lepidoptera and Trichoptera, the female is the homogametic sex (i.e., having two X sex chromosomes) and the male, heterogametic (XY or XO). Unfertilized eggs, therefore, will contain only X chromosomes. However, whether they contain one or two such chromosomes and, therefore, the sex of parthenogenetic offspring, depends on the behavior of the chromosomes during meiosis in the oocyte nucleus (Suomalainen, 1962; White, 1973).

Two forms of female-producing parthenogenesis (thelytoky) are known. In some species no meiotic division occurs during oogenesis. Therefore, offspring are diploid and female (ameiotic or apomictic parthenogenesis) and will have the same genetic makeup as the mother, unless mutation or insertion of transposable elements occurs (Heming, 2003). In meiotic (automictic) parthenogenesis, the typical reduction division is followed by nuclear fusion so that a diploid chromosome complement is retained. Again, therefore, the offspring are female but they have a different genetic make up from their mother.

Haploid parthenogenesis (arrhenotoky), where the oocyte nucleus undergoes meiosis that is not followed by nuclear fusion, is of relatively rare occurrence, though typical of Hymenoptera, Thysanoptera, and some homopterans and Coleoptera. It results in the production of males. In Hymenoptera, haploid parthenogenesis is facultative; that is, a female determines whether or not an egg will be fertilized. In the honey bee, for example, a queen normally lays fertilized eggs that develop into workers (diploid females). However, under certain conditions, for example, when the hive is crowded, and the workers construct larger than normal drone cells on the honeycomb, she will lay unfertilized eggs from which haploid males develop, as a preliminary to swarming.

Parthenogenesis, producing in most species female offspring, may confer two advantages. In a species whose population density may be (temporarily) low, the ability of an isolated female to reproduce parthenogenetically may ensure survival of her genotype until the population density increases and males are again likely to be encountered. More often, however, parthenogenesis is employed as a mechanism that provides a rapid mode of reproduction, to enable a species to take full advantage of temporarily ideal conditions. Thus, a parthenogenetic female, who does not require to locate, or be located by, a male can devote her time and energy to egg production. Further, all her offspring are female, so that her maximum reproductive potential can be realized. The disadvantage of parthenogenesis is that the genotype of successive generations remains more or less constant so that adaptation of a species to changing environmental conditions is very slow. To counteract this, many species alternate one or more parthenogenetic generations with a normal sexual generation. Aphids, for example, reproduce for most of the year by ameiotic parthenogenesis (Figure 8.8). However, toward fall (and affected by changing environmental conditions) there occurs, during maturation of some oocytes, a separation of the two X chromosomes,

614 one of which migrates to the polar body and is destroyed. From such eggs (with an XO

constitution) males will develop. As spermatogenesis occurs in these individuals, sperma-

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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