Giant Sperm

Giant Sperm

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A dotted line shows the site of the sperm-moving structure called a Zenker organ in this microscope view of a Pseudocandona marchica ostracod. This tiny crustacean, a relative of shrimps and crabs, uses the organ to route extralong sperm.
s. Yamada and R. Matz ke-Karasz/ Naturwissenschaften 2012
Crustaceans called ostracods face an unusual challenge: Their sperm can be up to 10 times as long as their bodies.
Admittedly, an ostracod’s body fits on the head of a pin, and the longest sperm filaments stretch only about a centimeter. Still, the mismatch intrigues biologists of a species that would have to produce sperm more than 15 meters long to reproduce in ostracod style. Now, two scientists have tackled the question of how ostracods’ internal plumbing handles such extreme sperm.
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Part of the ostracod sperm duct has evolved into a segment called a Zenker organ, toughened with crabshell-like chitin in ornate shapes. Detailed microscopy now suggests how Zenker organs work as pumps for giant sperm, says Shinnosuke Yamada of Shizuoka University in Japan.
In the freshwater ostracod Pseudocandona marchica , with sperm about half its body length, the Zenker organ looks like a tree trunk wearing wide ruffs of fringe. A narrow tube running down the center has an opening so narrow that only one sperm can pass through at a time, Yamada and Renate Matzke-Karasz of Ludwig Maximilians University in Munich report in the July Naturwissenschaften . The organ contracts and the tip of a sperm edges through a valve opening into the central tube, then muscles squeeze again and shorten the organ. When the muscles relax, the Zenker organ springs back to length, sliding the open valve a little way along the sperm filament. The whole process works a bit like a mechanical pencil ejecting more lead with each click.
But a Zenker organ is short and an ostracod’s sperm is long, so releasing even one sperm takes multiple contractions. During mating, P. marchica eventually delivers about a dozen sperm.
Giant sperm appear in various other species, including some flatworms, beetles and a fruit fly species, Drosophila bifurca, with sperm nearly 6 centimeters long. It’s not entirely clear how these species evolved outsized sperm, but researchers suspect it can involve both male competition and female favoritism for longer sperm, taken to extremes.
Questions or comments on this article? E-mail us at feedback@sciencenews.org
Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.
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Scientists Discover Ancient Fossilized Giant Sperm

Scientists Discover Ancient Fossilized Giant Sperm

Scientists say they have found what may be the oldest specimen of fossilized sperm ever discovered, inside a tiny crustacean trapped in a piece of amber 100 million years ago.
The researchers say the discovery in amber from Myanmar's Kachin province, described in a paper published Wednesday in the science journal Proceedings of the Royal Society of Biological Sciences , provides an extremely rare opportunity to study the evolution of the reproductive process.
The scientists suspect the crustacean in which the sperm was found, a newly discovered species of ostracod about 1 millimeter long, was likely covered in amber shortly after mating.
They say the sperm cell found in the animal was significant, not only because of the age of the specimen but also because of its size — about one-fifth the size of the entire animal that produced it.
The researchers say that while most animals produce huge numbers of tiny sperm, there are still animals that exist today that produce so-called “giant” sperm. Some modern ostracods and species of fruit flies produce sperm many times longer than their bodies.
One of the authors of the study, the University of Munich’s Renate Matzke-Karasz, says the most significant aspect of the discovery is that it shows this method of reproduction has been around a very long time.
The researchers say it is unclear what evolutionary advantage producing a small number of giant sperm, as opposed to a large number of tiny sperm, may have. While a large sperm might have a better chance of reaching an egg, the reproductive organs of the animal producing them must be large as well, which would require a lot of “biological energy.”
Matzke-Karasz says that before this discovery, evolutionary scientists questioned whether animals that developed this type of reproductive system were doomed to extinction. Now, she says, they know they can exist for millions of years.

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... Assuming trade-offs between gamete size and number, disruptive selection is hypothesized to lead to the evolution of males that produce many, small sperm that compete to fertilize eggs and females that produce fewer, larger eggs that are better able to provision zygotes 1,3,4 . Perhaps surprisingly then, sperm are the most diverse cell type known 5 , exhibiting tremendous variation in size across animals [5] [6] [7] . Sperm size diversification must therefore be attributed to factors beyond their common functional purpose of fertilizing eggs. ...
... We combined this dataset with a phylogeny spanning animal evolution generated from the Open Tree of Life 39 , which we time-calibrated using nodal divergence dates from TimeTree 40 . This dataset exemplifies the extraordinary diversity in sperm length observed in animals, with sperm ranging from an average of 2 μm in the rotifer Brachionus bidentatus 41 to nearly 6 cm in the fruit fly Drosophila bifurca 6 , representing four orders of magnitude difference in length. This pattern of diversity in sperm length was evident in every phylum examined (Supplementary Table 1). ...
Evolutionary biologists have endeavoured to explain the extraordinary diversity of sperm morphology across animals for more than a century. One hypothesis to explain sperm diversity is that sperm length is shaped by the environment where fertilization takes place (that is, fertilization mode). Evolutionary transitions in fertilization modes may transform how selection acts on sperm length, probably by affecting postcopulatory mechanisms of sperm competition and the scope for cryptic female choice. Here, we address this hypothesis by generating a macro-evolutionary view of how fertilization mode (including external fertilizers, internal fertilizers and spermcasters) influences sperm length diversification among 3,233 species from 21 animal phyla. We show that sperm are shorter in species whose sperm are diluted in aquatic environments (that is, external fertilizers and spermcasters) and longer in species where sperm are directly transferred to females (that is, internal fertilizers). We also show that sperm length evolves faster and with a greater number of adaptive shifts in species where sperm operate within females (for example, spermcasters and internal fertilizers). Our results demonstrate that fertilization mode is a key driver in the evolution of sperm length across animals, and we argue that a complex combination of postcopulatory forces has shaped sperm length diversification throughout animal evolution.
... Further, it is suggested that adult life-history traits are primarily influenced by the energy reserves that are believed to be static throughout the adult life (Roff 1992), perhaps a belief strengthened due to its positive relation with body size (Zwaan et al. 1995;Nunney 1996;Chippindale et al. 1997a;Prasad et al. 2000Prasad et al. , 2001. In a study involving 42 Drosophila species, RM in males was shown to be positively correlated with body size and sperm length, while in females RM was not correlated with body size (Pitnick et al. 1995) . In Drosophila melanogaster, a key model organism used in understanding many adaptive processes, large males attracted and acquired significantly more mates compared to smaller males due to production of louder and better quality courtship song (Partridge and Farquhar 1983;Partridge et al. 1987). ...
... A study with stalk-eyed fly, Cyrtodiopsis dalmanni showed RM in males to be negatively correlated with AG size (Baker et al. 2003). Another study involving 42 species of Drosophila showed RM to be positively correlated with testis size and sperm length (Pitnick et al. 1995) and testis mass shows a positive relationship with body mass (Pitnick 1996), while body mass is reported to be correlated with body size (Prasad et al. 2000;Prasad and Joshi 2003). Body size is a complex, quantitative phenotypic trait (Blanckenhorn 2000) that is suggested to be the most comprehensive predictor of fitness, especially in Drosophila fruit flies (for a complete list, see table 1 of Pavkovic-Lucic and Kekic 2013). ...
... Moreover, male meiotic spindles are substantially larger than the spindles of somatic cells and those of mammals. The sperm tail length in Drosophilids is extremely variable as well, ranging from that of D. persimilis (0.32 mm) to the giant~60 mm sperm produced by D. bifurca [131, 132], a whopping~185x ratio. Several studies aimed to understand the evolutionary strategy behind this difference in size, with the notion that longer sperm tails are inversely correlated with sperm numbers [133] and directly with the timing of male maturity [132]. ...
Drosophila dividing spermatocytes offer a highly suitable cell system in which to investigate the coordinated reorganization of microtubule and actin cytoskeleton systems during cell division of animal cells. Like male germ cells of mammals, Drosophila spermatogonia and spermatocytes undergo cleavage furrow ingression during cytokinesis, but abscission does not take place. Thus, clusters of primary and secondary spermatocytes undergo meiotic divisions in synchrony, resulting in cysts of 32 secondary spermatocytes and then 64 spermatids connected by specialized structures called ring canals. The meiotic spindles in Drosophila males are substantially larger than the spindles of mammalian somatic cells and exhibit prominent central spindles and contractile rings during cytokinesis. These characteristics make male meiotic cells particularly amenable to immunofluorescence and live imaging analysis of the spindle microtubules and the actomyosin apparatus during meiotic divisions. Moreover, because the spindle assembly checkpoint is not robust in spermatocytes, Drosophila male meiosis allows investigating of whether gene products required for chromosome segregation play additional roles during cytokinesis. Here, we will review how the research studies on Drosophila male meiotic cells have contributed to our knowledge of the conserved molecular pathways that regulate spindle microtubules and cytokinesis with important implications for the comprehension of cancer and other diseases.
... Sperm are one-half of the story of life for sexually reproducing animals, for which the fusion of sperm and eggs is necessary for the production of offspring. Yet, despite their shared function of fertilizing eggs, sperm are the most diverse cell type known, exhibiting large variation in size across animals, including examples of sperm 'gigantism [1] [2][3][4] ' . A range of hypotheses have been developed to explain the tremendous diversity in sperm morphology. ...
Sperm are the most morphologically variable cell type known, despite performing the same functional role of fertilizing eggs across all sexually reproducing species. Sperm morphology commonly varies among individuals, populations, closely related species, and across animal phyla. Sperm morphology has long been used as a tool for placing species in a phylogenetic context and a range of selective forces are hypothesized to influence sperm evolution and diversification. However, we currently lack robust examinations of macroevolutionary (i.e. across phyla) patterns of sperm evolution, due largely to the challenges of comparing sperm morphological data across the animal tree of life. Here we describe the SpermTree database, which currently represents 5,675 morphological descriptions of sperm morphology from 4,705 unique species from 27 animal phyla. This dataset includes measurements of sperm head, midpiece, flagellum and total length, the latter of which spans four orders of magnitude. All entries in the dataset are matched to currently accepted scientific names in taxonomic databases, facilitating the use of these data in analyses examining sperm evolution in animals.
... The experimental system for which the evolution of sperm form has been most intensively investigated is the fruit fly, Drosophila melanogaster, and its relatives. Comparative analyses, quantitative genetics, experimental evolution, and functional analyses have all provided complimentary demonstrations that the length of the female's primary sperm-storage organ, the seminal receptacle (SR), generates selection on sperm length, thus contributing to the co-diversification of these functionally interacting, sex-specific traits [18] [19] [20][21][22][23][24]. This selective process presumably underlies the multiple independent evolutionary origins of giant sperm across the Drosophila phylogeny [20,25]. ...
Postcopulatory sexual selection is credited as a principal force behind the rapid evolution of reproductive characters, often generating a pattern of correlated evolution between interacting, sex-specific traits. Because the female reproductive tract is the selective environment for sperm, one taxonomically widespread example of this pattern is the co-diversification of sperm length and female sperm-storage organ dimension. In Drosophila, having testes that are longer than the sperm they manufacture was believed to be a universal physiological constraint. Further, the energetic and time costs of developing long testes have been credited with underlying the steep evolutionary allometry of sperm length and constraining sperm length evolution in Drosophila. Here, we report on the discovery of a novel spermatogenic mechanism—sperm cyst looping—that enables males to produce relatively long sperm in short testis. This phenomenon (restricted to members of the saltans and willistoni species groups) begins early during spermatogenesis and is potentially attributable to heterochronic evolution, resulting in growth asynchrony between spermatid tails and the surrounding spermatid and somatic cyst cell membranes. By removing the allometric constraint on sperm length, this evolutionary innovation appears to have enabled males to evolve extremely long sperm for their body mass while evading delays in reproductive maturation time. On the other hand, sperm cyst looping was found to exact a cost by requiring greater total energetic investment in testes and a pronounced reduction in male lifespan. We speculate on the ecological selection pressures underlying the evolutionary origin and maintenance of this unique adaptation.
... Experimentally, more complex geometry has been examined, for example in sperm sorters (Denissenko et al., 2012;Tung et al., 2014;Kamal and Keaveny, 2018), though this complexity has inhibited numerical exploration of the same intricate environments. For instance, unexplored theoretically to the best of our knowledge, remarkable in vivo experiments of Yang and Lu (2011) exemplify the drastic effects that severe confinement can have on sperm motility in Drosophila, whose long sperm are able to move rapidly in the contorted female reproductive tract whilst being practically immobile in free artificial media (Pitnick et al., 1995; Lu, 2013). ...
In one of the first examples of how mechanics can inform axonemal mechanism, Machin's study in the 1950s highlighted that observations of sperm motility cannot be explained by molecular motors in the cell membrane, but would instead require motors distributed along the flagellum. Ever since, mechanics and hydrodynamics have been recognised as important in explaining the dynamics, regulation, and guidance of sperm. More recently, the digitisation of sperm videomicroscopy, coupled with numerous modelling and methodological advances, has been bringing forth a new era of scientific discovery in this field. In this review, we survey these advances before highlighting the opportunities that have been generated for both recent research and the development of further open questions, in terms of the detailed characterisation of the sperm flagellum beat and its mechanics, together with the associated impact on cell behaviour. In particular, diverse examples are explored within this theme, ranging from how collective behaviours emerge from individual cell
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