Diversity decline in mackerel sharks
Adult female great white shark, Carcharodon carcharias, a species of the shark order Lamniformes. Image courtesy of George T. Probst (photographer).
A multitude of factors, including variations in speciation and extinction rates, determine species richness, but the causes of lineage decline and disappearance are unclear. To better understand the diversity decline in mackerel sharks, Fabien Condamine et al. (pp. 20584–20590) analyzed diversification and extinction models and compared the fossil records of extant and extinct species of mackerel sharks with the fossil records of extant and extinct species of ground sharks. In the early Late Cretaceous, mackerel shark diversity peaked up to 3 times the current diversity. Over the last 20 million years, mackerel shark diversity declined. Younger mackerel shark species have a higher extinction rate than older species, and this extinction rate increased due to cooler temperatures during Cenozoic climate cooling as well as clade competition for resources. Cenozoic climate cooling may also have caused mackerel shark extinction to exceed speciation. Clade competition with ground sharks also influenced the diversification of medium and large mackerel sharks, though the largest mackerel sharks exhibited some of the lowest extinction and speciation rates. The results suggest that both abiotic and biotic factors influence the diversity of marine apex predators, according to the authors. — M.S.
Female orgasm and induced ovulation
Female orgasm is a complex neuro-endocrine process that is unlikely to have evolved by chance but is unnecessary for successful reproduction. Many hypotheses for the evolutionary origin of female orgasm have been proposed, but few have empirical support. Mihaela Pavlicev et al. (pp. 20267–20273) conducted an experimental test of one such hypothesis, the ovulatory homolog model (OHM), which proposes that the physiological mechanisms underlying female orgasm originally developed for inducing ovulation during copulation. Such copulation-induced ovulation (CIO) occurs in various mammals, such as rabbits, cats, ferrets, and camels, but not in humans or great apes. To test the OHM, the authors treated female rabbits daily with fluoxetine, a selective serotonin reuptake inhibitor that inhibits orgasm, for 2 weeks before copulation. One day after copulation, the authors measured the number of ovulations, which was 30% lower in fluoxetine-treated rabbits than in control rabbits. In a second experiment, the authors induced ovulation by injection of human chorionic gonadotropin (hCG) following fluoxetine treatment. Fluoxetine treatment did not significantly affect hCG-induced ovulation. According to the authors, the results support the hypothesis that CIO in rabbits is homologous to female orgasm in humans, suggesting that these processes share a common evolutionary origin. — B.D.
Genetic lineages of chytrid fungus
B. dendrobatidis.
The amphibian chytrid fungus Batrachochytrium dendrobatidis, also called Bd, is a devastating, globally distributed pathogen. Susceptible amphibians succumb to the fungus’ effects on skin functions, while resistant amphibians serve as reservoirs that spread the pathogen. Allison Byrne et al. (pp. 20382–20387) optimized a method to amplify and sequence noninvasive skin swab samples and characterize various genetic lineages of the chytrid fungus. The authors found that the globally distributed lineage BdGPL co-occurs with a previously unknown lineage, BdASIA3, which is widespread in Southeast Asia. The results also suggest that the range of the lineage BdCAPE has expanded and co-occurs with BdGPL. As lineages come into contact, genetic recombinations can create more virulent hybrids than parent lineages, resulting in additional threats to amphibians. The authors indicate the need for additional genetic data, particularly from locations where multiple chytrid fungus lineages come into contact. According to the authors, the optimized method used in this study suggests that archived skin swabs, rather than expensive field methods, can be used for the collection of future genetic data. — P.G.
How scaly-foot snails make natural semiconductors
Black scaly-foot snail.
Animal skeletons are hardened with oxygen-containing minerals, such as hydroxyapatite in mammal bones and teeth, calcium carbonate in mollusk shells, and silica in sponges. The scaly-foot snail Chrysomallon squamiferum, found in hydrothermal vents on the Indian Ocean’s Central Indian Ridge, is an exception, its scales and shell infused with iron sulfide nanoparticles that lend the snail its black metallic sheen. To uncover how the snail makes iron sulfide minerals, such as pyrite, greigite, and mackinawite, Satoshi Okada et al. (pp. 20376–20381) examined the scales of the black scaly-foot snail using mass spectrometry and electron microscopy. The analysis revealed that the snails accumulate sulfur through biological redox reactions in channel-like nanoscale columns that are continually constructed within the scales. When the authors placed scales from dead white scaly-foot snails, which inhabit a different area of the Central Indian Ridge and lack iron sulfide nanoparticles and the resultant black sheen, in hydrothermal vents inhabited by black scaly-foot snails, the translocated scales were covered with iron sulfide flakes within 13 days of incubation. The finding suggests that sulfur, provisioned by the snail, is combined with iron, sourced from the surrounding waters, to produce iron sulfide nanoparticles. Commercial production of pyrite, a semiconductor widely used in photodetectors, solar cells, and lithium batteries, requires high temperatures and expensive processes. According to the authors, the findings could lead to a biologically inspired alternative for the low-cost manufacture of iron sulfide nanoparticles such as pyrite. — P.N.