Sperm heteromorphism
Encyclopedia
Sperm heteromorphism is the simultaneous production of two or more distinguishable types of sperm
by a single male
. The sperm types might differ in size, shape and/or chromosome
complement. Sperm heteromorphism is also called sperm polymorphism or sperm dimorphism (for species with two sperm types). Typically, only one sperm type is capable of fertilizing eggs
. Fertile types have been called "eusperm" or "eupyrene sperm" and infertile types "parasperm" or "apyrene sperm".
One interpretation of sperm polymorphism is the "kamikaze sperm" hypothesis (Baker and Bellis, 1988), which has been widely discredited. The kamikaze sperm hypothesis states that the polymorphism of sperm is due to a subdivision of sperm into different functional groups. There are those that defend the egg from fertilization by other male sperm, and those that fertilize the egg. However, the polymorphism of sperm is probably only due to genetic variance not some organised military-like operation conducted by sperm. The sole purpose of every sperm is to impregnate the female and continue the proliferation of its own genes.
(i.e. the acquisition of fertilisation competence).
In silkworms, there is good evidence that fertile sperm are unable to fertilise if the non-fertile sperm are not present. The researchers artificially inseminated fertile sperm, non-fertile sperm or a mixture of both. Only the last group resulted in offspring production.
occurs. This might be offensively (e.g. displacing or even killing rival fertilising sperm) defensively (e.g. by blocking areas of the female tract or creating a hostile pre-fertilisation environment).
). For example, sperm might fill up the sperm storage organs so that female "perceives" that she does not need to re-mate to obtain more sperm. Alternatively, the sperm may transfer chemicals similar to sex peptide, a chemical carried on the sperm of Drosophila melanogaster
that makes females less likely to accept mates (i.e. it is an anti-aphrodesiac).
There is correlational evidence for this theory in a butterfly, Pieris napi. Females that were receptive to a second mating had fewer of the non-fertile sperm type in storage than did non-receptive females. Thus, the infertile sperm may be responsible for delaying female remating.
This theory was also tested in the fruit-fly Drosophila pseudoobscura, but the results suggested that "cheap filler" was not important in that species.
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
by a single male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
. The sperm types might differ in size, shape and/or chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
complement. Sperm heteromorphism is also called sperm polymorphism or sperm dimorphism (for species with two sperm types). Typically, only one sperm type is capable of fertilizing eggs
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
. Fertile types have been called "eusperm" or "eupyrene sperm" and infertile types "parasperm" or "apyrene sperm".
One interpretation of sperm polymorphism is the "kamikaze sperm" hypothesis (Baker and Bellis, 1988), which has been widely discredited. The kamikaze sperm hypothesis states that the polymorphism of sperm is due to a subdivision of sperm into different functional groups. There are those that defend the egg from fertilization by other male sperm, and those that fertilize the egg. However, the polymorphism of sperm is probably only due to genetic variance not some organised military-like operation conducted by sperm. The sole purpose of every sperm is to impregnate the female and continue the proliferation of its own genes.
Insects
- LepidopteraLepidopteraLepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...
(i.e. butterflies and moths): Almost all known speciesSpeciesIn biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
produce two sperm types. The fertilizing type has a longer tail and contains a nucleusCell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
. The other type is shorter and lacks a nucleus, meaning it contains no genetic information at all.
- DrosophilaDrosophilaDrosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
(fruit-flies): the D. obscura group of species in the genusGenusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Drosophila is sperm heteromorphic. As with the Lepidoptera, there is a long, fertile type and a short, infertile type. However, the infertile type has a nucleus with a normal, haploid chromosome complement. It is not known why the shorter sperm are infertile, though it has been suggested that the slightly wider head of the infertile type might prevent it from entering the micropyleMicropyleA micropyle is small opening in the surface of an ovule, through which the pollen tube penetrates, often visible as a small pore in the ripe seed....
of the egg.
- Diosidae (stalk-eyed flies): several species have a long, fertile type and a shorter infertile type.
- Carabidae (ground beetles): some species produce large, infertile sperm that may contain up to 100 sets of chromosomes.
Molluscs
- Some prosobranch gastropods (snails) produce two or three sperm types. The infertile types may be large "carrier" types the fertile sperm attach to for transport, or "lancet" types. The lancet sperm sometimes contain many lysosomes.
Fish
- Some SculpinSculpinA Sculpin is a fish that belongs to the order Scorpaeniformes, suborder Cottoidei and superfamily Cottoidea, that contains 11 families, 149 genera, and 756 species...
may be sperm heteromorphic. Their ejaculates appear to contain fertile sperm as well as disc-shaped, infertile sperm.
Non-adaptive
The non-fertilising morph(s) have no function, and are simply developmental errors. This is thought to be unlikely in many sperm heteromorphic species because the production of infertile sperm may be highly regulated, and infertile sperm can make up >90% of the total sperm in some Lepidoptera and Drosophila.Provisioning
The non-fertilising morph(s) are a means in which males can provide nutrition to the female, her eggs or the fertilising sperm.Facilitation
The non-fertilising sperm help the fertilising sperm by assisting their transport or capacitationCapacitation
Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte.This step is a biochemical event; the sperm move normally and look mature prior to capacitation....
(i.e. the acquisition of fertilisation competence).
In silkworms, there is good evidence that fertile sperm are unable to fertilise if the non-fertile sperm are not present. The researchers artificially inseminated fertile sperm, non-fertile sperm or a mixture of both. Only the last group resulted in offspring production.
Sperm competition
Non-fertile sperm increase the fertilisation success of the male producing them when sperm competitionSperm competition
Sperm competition is a term used to refer to the competitive process between spermatozoa of two different males to fertilize an egg of a lone female. Competition occurs whenever females engage in promiscuous mating to increase their chances in producing more viable offspring...
occurs. This might be offensively (e.g. displacing or even killing rival fertilising sperm) defensively (e.g. by blocking areas of the female tract or creating a hostile pre-fertilisation environment).
"Cheap filler"
Non-fertile sperm delay or prevent the female mating again, thus allowing the male producing the non-fertile sperm a greater share of the paternity of her offspring (because the male avoids Sperm competitionSperm competition
Sperm competition is a term used to refer to the competitive process between spermatozoa of two different males to fertilize an egg of a lone female. Competition occurs whenever females engage in promiscuous mating to increase their chances in producing more viable offspring...
). For example, sperm might fill up the sperm storage organs so that female "perceives" that she does not need to re-mate to obtain more sperm. Alternatively, the sperm may transfer chemicals similar to sex peptide, a chemical carried on the sperm of Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
that makes females less likely to accept mates (i.e. it is an anti-aphrodesiac).
There is correlational evidence for this theory in a butterfly, Pieris napi. Females that were receptive to a second mating had fewer of the non-fertile sperm type in storage than did non-receptive females. Thus, the infertile sperm may be responsible for delaying female remating.
This theory was also tested in the fruit-fly Drosophila pseudoobscura, but the results suggested that "cheap filler" was not important in that species.