05/25/2026
Now ain’t this a thing! Fish in the news trapping a holiday in a worn hole…
Here, researchers investigated an unusual reproductive event in the normally biparental cichlid fish 𝐶𝑦𝑝ℎ𝑜𝑡𝑖𝑙𝑎𝑝𝑖𝑎 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎, in which a female produced offspring without a male. Using whole-genome sequencing data, the authors analysed whether reproduction occurred via selfing or parthenogenesis by comparing patterns of heterozygosity with those from a wild, genetically diverse 𝐶𝑦𝑝ℎ𝑜𝑡𝑖𝑙𝑎𝑝𝑖𝑎 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 family collected in Lake Tanganyika and a closely related in**ed 𝐶𝑡𝑒𝑛𝑜𝑐ℎ𝑟𝑜𝑚𝑖𝑠 𝑏𝑒𝑛𝑡ℎ𝑖𝑐𝑜𝑙𝑎 family.
The 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 findings contribute to a broader understanding of how alternative reproductive strategies evolve in vertebrate lineages.
Extract from paper - "The regular mode of s*xual reproduction is biparental in vertebrates and is also predominant for cichlids as well as the here investigated species 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎. The percentage of heterozygous sites (Table S1) was higher in the wild 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 parental individuals (20.10 ± 2.85%) and the uniparental female (15.42%) than in the in**ed 𝐶𝑡. 𝑏𝑒𝑛𝑡ℎ𝑖𝑐𝑜𝑙𝑎 parents (5.71 ± 0.08%), supporting that biparental s*xual reproduction is the predominant reproductive mode in 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎, with selfing representing a facultative mode under certain environmental conditions or reproductive constraints.
There are a few reported cases of selfing in fishes, specifically, in the mangrove killifish 𝐾𝑟𝑦𝑝𝑡𝑜𝑙𝑒𝑏𝑖𝑎𝑠 𝑚𝑎𝑟𝑚𝑜𝑟𝑎𝑡𝑢𝑠 and 𝐾. ℎ𝑒𝑟𝑚𝑎𝑝ℎ𝑟𝑜𝑑𝑖𝑡𝑢𝑠, and in the cichlid fish 𝐵𝑒𝑛𝑖𝑡𝑜𝑐ℎ𝑟𝑜𝑚𝑖𝑠 𝑛𝑖𝑔𝑟𝑜𝑑𝑜𝑟𝑠𝑎𝑙𝑖𝑠. In natural populations of 𝐾. 𝑚𝑎𝑟𝑚𝑜𝑟𝑎𝑡𝑢𝑠, males are rare and many individuals are simultaneous hermaphrodites that reproduce via selfing. In cichlids, s*x determination is controlled by highly dynamic s*x chromosomes, and s*x change can occur in some species. However, whether 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 is capable of s*x change during its lifetime is so far unknown. Moreover, Svensson et al., 2016 reported a functional hermaphrodite first-generation hybrid capable of selfing, obtained by crossing two s*xually reproducing 𝑃𝑢𝑛𝑑𝑎𝑚𝑖𝑙𝑖𝑎 species. Therefore, selfing may be possible in cichlids under certain conditions. Notably, natural hybridization has been observed along a narrow hybrid zone between the northerly distributed 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 and its southern sister species 𝐶𝑦. 𝑔𝑖𝑏𝑏𝑒𝑟𝑜𝑠𝑎. Yet, our uniparental 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 female shows heterozygosity estimates comparable to those of the wild parents, originating from outside the hybrid zone, making a hybrid origin unlikely as elevated heterozygosity would be expected under recent hybridization.
By using selfing as an adaptive strategy, organisms can assure reproduction when males are unavailable. In contrast to as*xual reproduction, selfing may also be more efficient to purge deleterious recessive mutations. Facultative selfing is therefore expected to be favoured in systems where mating opportunities are unpredictable or spatially constrained. In cichlids, such conditions may be influenced by life-history traits and temporal or spatial environmental heterogeneity shaping reproductive opportunities. 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 is one of the larger Lake Tanganyika cichlids, reaching a total length of 35 cm, and inhabits deeper rocky habitats where it typically occurs in social groups consisting of a dominant male and multiple females. Such a social structure, combined with spatial segregation in patchily distributed habitats in the northern part of the lake, may lead to periods of limited mate access for some individuals.
In addition, 𝐶𝑦. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 is a maternal mouthbrooder with a long brooding period of approximately 40 days and low fecundity, increasing costs of reproductive failure when mating opportunities are rare or unpredictable. Under these conditions, facultative selfing could represent an alternative reproductive strategy that ensures reproductive output when males are unavailable, particularly in isolated or low density habitats. Beyond these short-term fitness benefits, reproductive flexibility may also have longer-term evolutionary consequences. Given that variation in life-history traits is discussed to contribute to diversification in adaptive radiations, our results suggest that such reproductive flexibility may constitute an additional mechanism facilitating rapid speciation.
Future studies should therefore address two complementary questions: under which ecological and social conditions – such as low population density, reduced habitat connectivity, or skewed s*x ratios – alternative reproductive modes are expressed, and whether such flexibility ultimately promotes or constrains diversification in adaptive radiations."
* Correction to above extract, noted by Alan Smith in comments - In the Svenson et al paper the selfing fish was a 𝑃𝑢𝑛𝑑𝑎𝑚𝑖𝑙𝑖𝑎 x 𝑁𝑒𝑜𝑐ℎ𝑟𝑜𝑚𝑖𝑠 hybrid, not two 𝑃𝑢𝑛𝑑𝑎𝑚𝑖𝑙𝑖𝑎 species as stated.
𝗥𝗲𝘀𝗲𝗮𝗿𝗰𝗵 𝗧𝗶𝘁𝗹𝗲
Genomic evidence for facultative selfing in the cichlid fish 𝐶𝑦𝑝ℎ𝑜𝑡𝑖𝑙𝑎𝑝𝑖𝑎 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎
Open-access (preprint) - https://www.biorxiv.org/content/10.64898/2026.05.13.724898v1
𝗪𝗵𝗮𝘁 𝗶𝘀 𝗮 𝗣𝗿𝗲𝗽𝗿𝗶𝗻𝘁 𝗣𝘂𝗯𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻?
https://www.biorxiv.org/content/what-unrefereed-preprint
𝗖𝗶𝘁𝗮𝘁𝗶𝗼𝗻
M. Efe Uysal, Daniela Souza-Costa, Allison Marks, Adrian Indermaur, Wolfgang Gessl, Walter Salzburger, Julia M. I. Barth, Genomic evidence for facultative selfing in the cichlid fish 𝐶𝑦𝑝ℎ𝑜𝑡𝑖𝑙𝑎𝑝𝑖𝑎 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎. bioRxiv 2026.05.13.724898; doi: https://doi.org/10.64898/2026.05.13.724898
𝗔𝗯𝘀𝘁𝗿𝗮𝗰𝘁
Organisms have evolved a remarkable diversity of reproductive strategies in response to environmental variations and selective pressures. Although most vertebrates do reproduce biparentally, rare alternative modes such as selfing (self-fertilization) and different forms of parthenogenesis exist, but remain poorly characterized. Here, we investigated an unusual reproductive event in the normally biparental cichlid fish 𝐶𝑦𝑝ℎ𝑜𝑡𝑖𝑙𝑎𝑝𝑖𝑎 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎, in which a female produced offspring in the absence of a male. Using whole-genome sequencing data, we analyzed whether reproduction occurred via selfing or parthenogenesis by comparing patterns of heterozygosity with those from a wild, genetically diverse 𝐶. 𝑓𝑟𝑜𝑛𝑡𝑜𝑠𝑎 family collected in Lake Tanganyika and a closely related in**ed 𝐶𝑡𝑒𝑛𝑜𝑐ℎ𝑟𝑜𝑚𝑖𝑠 𝑏𝑒𝑛𝑡ℎ𝑖𝑐𝑜𝑙𝑎 family. The uniparental family exhibited reduced genetic diversity, elevated relatedness, and genome-wide patterns of homozygosity distinct from those expected under parthenogenesis or inbreeding, but consistent with self-fertilization. Our study provides rare genomic evidence of selfing in a vertebrate and suggests that such alternative reproductive modes may be overlooked rather than truly absent. These findings contribute to a broader understanding of how alternative reproductive strategies evolve in vertebrate lineages.
𝗦𝗶𝗴𝗻𝗶𝗳𝗶𝗰𝗮𝗻𝗰𝗲 The overwhelming majority of vertebrates reproduce s*xually, requiring a male and a female to produce genetically distinct offspring. Yet, rare alternative modes involving only a single parent such as as*xual parthenogenesis (“virgin birth”) or self-fertilization challenge this paradigm. Among these, selfing is exceptionally uncommon and poorly studied in vertebrates. Here, we unveiled - based on genomic analyses - the reproductive strategy of a member of the extraordinarily diverse cichlid fish radiation in Lake Tanganyika that reproduced in captivity in the absence of a male. By comparing patterns of genome-wide heterozygosity with both wild and in**ed reference families, we identified a rare case of selfing. This finding adds to the limited records of selfing in vertebrates and expands current understanding of reproductive diversity, highlighting the power of whole-genome sequencing to distinguish among alternative reproductive mechanisms.
𝗣𝗵𝗼𝘁𝗼 𝗖𝗿𝗲𝗱𝗶𝘁
Top - Portrait photo of male 𝐶𝑦𝑝ℎ𝑜𝑡𝑖𝑙𝑎𝑝𝑖𝑎 𝑠𝑝. (aquarium raised). Shutterstock stock photo, cropped.
Bottom - During canonical meiosis, DNA replication is followed by two successive divisions (meiosis I and II) to produce haploid gametes (a). Diploidy is restored in s*xual reproduction either by fusion of gametes from two individuals (outcrossing), maintaining genome-wide heterozygosity, or by fusion of two independent gametes from the same individual (selfing), resulting in genome-wide reduction and stochastic redistribution of heterozygosity (b). In as*xual (parthenogenetic) reproduction, diploidy is restored without fertilisation, either via mitotic divisions (apomixis) producing clonal offspring with fully retained heterozygosity, or via altered meiosis (automixis) (c). Automixis comprises distinct cytological mechanisms with characteristic genomic outcomes: Fusion of meiotic products (d) can occur after meiosis I (central fusion), reuniting homologs and retaining high heterozygosity, particularly in regions of low recombination (e.g., near centromeres), or after meiosis II (terminal fusion), where fusion of sister chromatids results in extensive homozygosity. Alternatively, in automixis, diploidy can be restored through modification of meiotic division itself (e): Suppression of meiosis I (first division restitution, FDR) prevents homolog segregation and largely preserves heterozygosity, whereas suppression of meiosis II (second division restitution, SDR) allows homolog segregation but retains sister chromatids, resulting in partial loss of heterozygosity with recombination-dependent patterns. In premeiotic endoreplication, genome duplication prior to meiosis leads to pairing of identical copies, effectively bypassing homolog interactions and preserving heterozygosity. Finally, postmeiotic genome duplication (gamete duplication) (f) restores diploidy by doubling a haploid genome, resulting in complete homozygosity. Percentages display retained heterozygosity (H).
© 2026 the Author(s). Published on bioRxiv Preprint Server. This paper is released under a Creative Commons Attribution 4.0 International (CC-BY-4.0) licence.
*xualParthenogenesis
