These female fish do something extremely rare in vertebrates; they produce offspring without a male

In most animals, reproduction requires two parents — a male and a female — each contributing half their genetic material through sex cells called gametes. However, in self-fertilization, or selfing, both sex cells come from the same individual. It is still a form of sexual reproduction, but uniparental — meaning a single organism produces both the egg and the sperm that fuse to create offspring. Because selfing is an extreme form of inbreeding, it is expected to reduce an offspring’s genetic variation by roughly 50% with each generation, according to the researchers.

Among vertebrates — the group that includes fish, reptiles, birds, and mammals — “selfing is exceptionally uncommon and poorly studied,” researchers say in the paper. It is well-established in only a few vertebrate species, like the mangrove killifish (Kryptolebias marmoratus), a small fish from the Americas, in which many individuals are simultaneous hermaphrodites that routinely self-fertilize because males are scarce. In cichlid fish specifically, two cases have been reported — one in the species Benitochromis nigrodorsalis, and one in a laboratory-produced hybrid between two Pundamilia cichlids.

Now, Julia Barth at the University of Basel in Switzerland and her colleagues have added another case, this time in the Humphead cichlid (Cyphotilapia frontosa), a large, deep-bodied species native to Lake Tanganyika in East Africa. Their findings, posted as a preprint in bioRxiv in May 2026, are based on whole-genome sequencing of a captive female and her six offspring. The researchers write that their study “provides rare genomic evidence of selfing in a vertebrate and suggests that such alternative reproductive modes may be overlooked rather than truly absent.”

The female Humphead cichlid was kept by a co-author of the study as the sole individual of her species in an aquarium for nine years. Her tank contained five clown loaches and one pleco. The fish from orders so distantly related to cichlids that any genetic contribution from them was not possible. In 2022, she produced six offspring with no male cichlid present at any point.

Cichlids using self-fertilization

To investigate how this happened, the team sequenced the full genomes of the mother and all six offspring, then compared these against two reference families: a wild Humphead cichlid family collected from Lake Tanganyika in Tanzania, and a closely related but heavily inbred aquarium family of Ctenochromis benthicola. The comparison allowed them to test whether the offspring had any paternal genetic contribution, and to narrow down which reproductive mechanism was responsible.

The first question was whether a male had contributed genetically at all. The team measured the rate of Mendelian violations — genotypes in the offspring that cannot be explained if the mother is the only parent. In the wild biparental family, ignoring the father’s genotypes produced a Mendelian violation rate of around 9%. In the uniparental family, the same analysis yielded a rate of just 0.05%, in line with background levels from genotyping errors and natural mutations. The researchers say this difference supports the absence of any paternal contribution.

With paternal involvement ruled out, the team turned to distinguishing between possible uniparental mechanisms. Different reproductive strategies leave characteristic patterns of retained heterozygosity — the degree to which offspring inherit both versions of each gene from the mother. In mitotic parthenogenesis, offspring are essentially genetic copies of the mother and retain nearly all her heterozygosity. In gamete duplication, the offspring’s genome is a doubled-up version of a single haploid cell, resulting in near-complete homozygosity. The offspring of the lone female showed genome-wide heterozygosity retention of around 50%. This means that the reproduction strategy used by the cichlid was neither of the above.

The researchers then examined how heterozygosity was distributed across individual chromosomes. In certain forms of parthenogenesis, the regions near centromeres, the midpoints of chromosomes, should show predictable and consistent patterns of heterozygosity or homozygosity across the genome.

Across 16 chromosomes with known centromere positions, heterozygosity retention ranged from nearly 0% to nearly 100% within the same offspring, with no uniform state shared across chromosomes or siblings. The researchers say this variable, chromosome-by-chromosome pattern is consistent with selfing, in which gametes are produced through independent meiotic events and fuse in ways that differ from one offspring to the next.

Selfing isn’t usual

Under natural conditions, the Humphead cichlids breed sexually in pairs. The species reaches around 35 centimeters in length, lives in deeper rocky habitats of Lake Tanganyika, and typically forms social groups consisting of one dominant male and several females. It is a maternal mouthbrooder, carrying and protecting developing eggs in the mouth for roughly 40 days, and it has low overall fecundity. The researchers note that these life-history characteristics — combined with the species’ spatially patchy, deep-water habitat — may create conditions where access to mates is sometimes limited.

“Facultative [optional] selfing could represent an alternative reproductive strategy that ensures reproductive output when males are unavailable, particularly in isolated or low-density habitats,” researchers say. The researchers are not suggesting the species regularly reproduces this way, but that the capacity may exist as an occasional alternative when normal mating is not possible.

Whether the female underwent some form of physiological change — such as sex reversal or hermaphroditism — to produce functional sperm remains an open question the study does not resolve. The researchers note that sex determination in cichlids involves highly dynamic sex chromosomes and that sex change occurs in some cichlid species, but whether it occurs in Humphead cichlids is unknown.

Compared to asexual reproduction, the researchers note that selfing may carry one biological advantage. “It may also be more efficient to purge deleterious recessive mutations,” they say, because natural selection can act on harmful variants that are exposed in homozygous offspring.