Sperm heteromorphism is the simultaneous production of two or more distinguishable types of

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

by a single

male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...

. The sperm types might differ in size, shape and/or

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

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 Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

. 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 in humans. 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, there is no evidence that the polymorphism of human sperm is for the purpose of antagonizing rival sperm.

Distribution

Sperm heteromorphism is known from several different groups of animals.

Insects

Lepidoptera Lepidoptera ( ) is an order (biology), order of insects that includes butterfly, butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 Family (biology), families and 46 Taxonomic r ...

(i.e. butterflies and moths): Almost all known

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

produce two sperm types. The fertilizing type has a longer tail and contains a

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

. The other type is shorter and lacks a nucleus, meaning it contains no genetic information at all. *

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...

(fruit-flies): the ''D. obscura'' group of species in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

''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 Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

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

micropyle Micropyle may refer to: * Micropyle (botany) In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (o ...

of the egg. * Diosidae (stalk-eyed flies): several species have a long, fertile type and a shorter infertile type. *

Carabidae Ground beetles are a large, cosmopolitan family of beetles, the Carabidae, with more than 40,000 species worldwide, around 2,000 of which are found in North America and 2,700 in Europe. As of 2015, it is one of the 10 most species-rich animal fami ...

(ground beetles): some species produce large, infertile sperm that may contain up to 100 sets of chromosomes.

Molluscs

*Some

prosobranch Prosobranchia was a large taxonomic subclass of sea snails, land snails and freshwater snails. This taxon of gastropods dates back to the 1920s. It has however been proven to be polyphyletic (consisting of more than one lineage of descent). Gene ...

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. Ther ...

(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 A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane prote ...

Fish

*Some

Sculpin A sculpin is a type of fish that belongs to the superfamily Cottoidea in the order Scorpaeniformes.Kane, E. A. and T. E. Higham. (2012)Life in the flow lane: differences in pectoral fin morphology suggest transitions in station-holding demand a ...

may be sperm heteromorphic. Their ejaculates appear to contain fertile sperm as well as disc-shaped, infertile sperm.

Possible functions of sperm heteromorphism

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 sexual transport or

capacitation 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 ...

(i.e. the acquisition of fertilisation competence). This has been demonstrated in ''Bombyx mori.'' 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 competition Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and ...

occurs. This might be offensive (e.g. displacing or even killing rival fertilising sperm) or defensive (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 competition Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and ...

). 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'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...

'' 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.Swallow, J.G., and G.S. Wilkinson. 2002. The long and the short o
sperm polymorphisms in insects
Biological Reviews 77: 153-182.

References

* Till-Bottraud, I., D. Joly, D. Lachaise and R.R. Snook. 2005
Pollen and sperm heteromorphism: convergence across kingdoms?
Journal of Evolutionary Biology 18(1): 1-18. Reproductive system