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In parthenogenesis eggs may develop in any of three ways: (1) they may begin as tetraploid or diploid bodies which undergo reduction in chromosome number as if in preparation for fertilization; but if fertilization is lacking, the males developing from them have the reduced or haploid set of hereditary factors; (2) unfertilized eggs may start as haploids and subsequently acquire the diploid number of chromosomes in some stage of cleavage, or (3) unfertilized eggs may start and end as diploids. In Hymenoptera diploids are usually females, all normal males originate as haploids. There are some exceptions, which will be discussed later. (Also see Gordh et al. 1999).
The end result of all parthenogenetic ontogeny (development) is a fatherless or impaternate animal. These, of course have grandfathers! In Types 2 and 3 parthenogenesis above such animals are diploid. In Type 1 we have impaternate haploids which pose several problems such as (a) their survival with a reduced chromosome number, (b) sex determination and (c) spermatogenesis in the haploid male.
Except for those haploid males regularly produced in six or seven groups of invertebrates, there have been very few known haploid adult animals. On the other hand, diploid and polyploid impaternates are known to occur with some frequency and as independent events in most of the larger groups of Metazoa. Sex determination in these larger Metazoan groups is entirely orthodox, while that of haploid males involves an entirely different genetic mechanism. Different kinds of animals combine different types of parthenogenesis with bisexual reproduction in their life cycles in various complicated ways.
Parthenogenesis may be natural or it may be artificial, induced by some artificial stimulus. It may be incomplete (rudimentary), the embryo dying before maturity, or it may be complete, leading to viability as adults. It may be obligatory, occurring from a type of egg that cannot normally be fertilized, or it may be facultative if the egg can develop with or without fertilization.
Considering the sex of the impaternate offspring, parthenogenesis includes arrhenotoky (production of impaternate males), thelytoky (production of impaternate females) and deuterotoky (production of both sexes parthenogenetically).
Parthenogenesis may be constant, occurring in each successive generation, or it may be cyclic in which case one or more parthenogenetic generations alternates with a bisexual. In cyclic parthenogenesis (heterogony), the agamic (or parthenogenetic) generation consists almost entirely of females. Individuals of the bisexual generation, both males and gamic females, are impaternate.
Parthenogenesis may occur as a general condition throughout the range of the species, or it may be geographic in which case the parthenogenetic form occupies a different area from the bisexual. Males may be absent or rare (spanandry) within the range of the parthenogenetic form.
There are two main cytological processes involved in parthenogenesis, apomictic and automictic. In apomixis there is one maturation division in the egg that is equational. There is no reduction in chromosomes so that the diploid number is maintained. Apomixis is considered the simplest type of parthenogenesis. Heterozygosity steadily increases in these species because when gene mutations and structural rearrangements occur, the heterozygosity is maintained in the following generation. Mutation cannot be homozygous and elimination of recessive mutations is impossible. This continued increase in heterozygosity allows for greater adaptiveness and dispersal through heterosis (White 1954, Smith 1955, Suomalinen 1962). Apomixis is a common name for uniparental procreation in which the sexual structures are retained (Dobzhansky 1941).
In automixis, the early stages of meiosis are similar to biparental species in the production of a haploid oocyte through reduction; however, a third division occurs resulting in a diploid. This restoration of the diploid number is accomplished in different ways in different species (Onions 1912, Whiting 1935, Speicher & Speicher 1938, Flanders 1945, Doutt & Smith 1950, S. G. Smith 1955, Tucker 1958, Bacci 1965).
Arrhenotoky vs Thelytoky
In arrhenotoky males are impaternate and females paternate. This is the most common type of parthenogenesis found in Hymenoptera. Actually female production is generally regarded as zygogenetic and not parthenogenetic. Fertilized eggs result in diploid females, while unfertilized eggs yield haploid males (Flanders 1939, White 1954, Bacci 1965). Several animal groups showing arrhenotoky are the Thysanoptera, rotifers, Coleoptera (Micromalthus), Acarina (all except the suborder Mesostigmata), Iceryini (cottony-cushion scale), and the Aleurodidae.
Thelytoky results in the production of impaternate females. Males are rare and are considered usually nonfunctional in reproduction, although in the laboratory they have been observed to function (Legner 1969, Rossler & DeBach 1972). Cytological processes may be either apomictic or automictic.
Deuterotoky does not differ from thelytoky other than males are more common. Some workers favor the elimination of this category entirely.
By 1940 Clausen listed 30 or more genera of parasitic Hymenoptera that were known with one or more species that reproduced uniparentally (by thelytoky). Today the number is much larger. Flanders (1945) regarded any biparental (arrhenotokous) population to be capable of thelytokous reproduction at times. He indicated the difficulties in distinguishing one from the other. He observed that the Cynipidae showed bisexuality most often, although the family usually reproduced unisexually. Speicher & Speicher (1938) noted that uniparental females of Bracon hebetor were obtained almost entirely from biparental females that resulted from crossing certain strains.
The difference between thelytoky and deuterotoky is sometimes confusing. Some parasitoids that were initially classified as thelytokous, have been found on detailed examination to produce an occasional son, although such sons are though to be nonfunctional (White 1984), which may be based on insufficient evidence (Marchal 1936, Flanders 1942, Wilson & Woolcock 1960, Bowen & Stern 1966, Birova 1970, Eskafi & Legner 1974, Laraichi 1978, Jardak et al. 1979, Stile & Davring 1980, Sorakina 1987). Males are often found in laboratory populations of thelytokous species and their frequency usually depends on the temperature at which their thelytokous mothers develop (Flanders 1942, Schlinger & Hall 1959, Flanders 1965, Eskafi & Legner 1974, Gordh & Lacey 1976, Laraichi 1978, Jardak et al. 1979, Cabello & Vargas 1985, Sorakina 1987, Luck et al. 1996).
Exercise 15.1--Distinguish zygogenesis from parthenogenesis.
Exercise 15.2--What are three possible fates of eggs in parthenogenesis?
Exercise 15.3--Discuss different manifestations of parthenogenesis.
Exercise 15.4--What principal cytological processes are involved in parthenogenesis? Discuss each.
Exercise 15.5--Distinguish arrhenotoky and thelytoky.
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