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

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), 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 ...

by a single

male Male (Planet symbols, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or Egg cell, ovum, in the process of fertilisation. A male organism cannot sexual repro ...

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

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

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 An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

. 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 ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...

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

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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 Nucleu ...

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

Drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...

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

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

''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 (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

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) a minute opening in the integument of an ovule of a seed plant. * Micropyle (zoology) A micropyle is a pore in the membrane covering the ovum, through which a sperm enters. Micropyles are also found in s ...

of the egg. *

Diopsidae Stalk-eyed flies are insects of the fly family Diopsidae. The family is distinguished from most other flies by most members of the family possessing "eyestalks": projections from the sides of the head with the eyes at the end. Some fly species fr ...

(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 ...

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

Molluscs

*Some

prosobranch Prosobranchia was a large Taxonomy (biology), taxonomic Class (biology), 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 polyphyly, polyphyletic (consis ...

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the 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 that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...

Fish

*Some

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

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 Spermatozoon, spermatozoa of two or more different males to fertility, fertilize the same Egg cell, egg during sexual reproduction. Competition can occur when females have multiple potential m ...

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 Spermatozoon, spermatozoa of two or more different males to fertility, fertilize the same Egg cell, egg during sexual reproduction. Competition can occur when females have multiple potential m ...

). 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 (an insect of the Order (biology), 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", "pomace fly" ...

'' 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