Precocious Sperm Exchange in the Simultaneously Hermaphroditic Nudibranch,Berghia stephanieae

Abstract

Sexual systems vary greatly across molluscs. This diversity includes simultaneous hermaphroditism, with both sexes functional at the same time. Most nudibranch molluscs are thought to be simultaneous hermaphrodites, but detailed studies of reproductive development and timing remain rare as most species cannot be cultured in the lab. The aeolid nudibranch, Berghia stephanieae, is one such species that can be cultured through multiple generations on the benchtop. We studied B. stephanieae reproductive timing to establish when animals first exchange sperm and how long sperm can be stored. We isolated age- and size-matched individuals at sequential timepoints to learn how early individuals exchange sperm. Individuals isolated at 10 weeks post initial feeding (wpf; ∼13 weeks post laying, wpl) can produce fertilized eggs. This is 6 weeks before animals first lay egg masses, indicating that sperm exchange occurs well before individuals are capable of laying eggs. Our results indicate that male gonads become functional for animals between 6 mm (∼6wpf, ∼9 wpl) and 9 mm (∼12wpf, ∼15 wpl) in length. That is much smaller (and sooner) than the size (and age) of individuals at first laying (12–19 mm; ∼16 wpf, ∼19 wpl), indicating that male and female functions do not develop simultaneously. We also tracked the number of fertilized eggs in each egg mass, which remained steady for the first 10–15 egg masses, followed by a decline to near-to-no fertilization. This dataset provides insights into precise timing of the onset of functionality of the male and female reproductive systems in B. stephanieae. These data contribute to a broader understanding of reproductive development and the potential for understanding evolution of diverse sexual systems in molluscs.

Egg-laying hormone expression in identified neurons across developmental stages and reproductive states of the nudibranchBerghia stephanieae

“…In the present study, sub-adults were communally housed and although they had not laid eggs yet and had no eggs visible in their reproductive organs when they were sacrificed, these animals likely had already mated. Precocious mating with early sperm donation, weeks prior to egg-laying, was recently reported in B. stephanieae (Taraporevala et al, 2022). We found that in sub-adults capable of this behavior (N=5), all 18 neuron classes seen in adults were identifiable (Figure 6A).…”

“…This is reminiscent of what was observed in L. stagnalis ; transcription of ELH in young animals builds until it is released by translation which happens at the time of egg-laying (De Lange et al, 1994). Behavioral work in B. stephanieae recently showed that even virgin B. stephanieae that were isolated their entire lives eventually laid eggs, albeit weeks later than animals in their cohort that had access to mates (Taraporevala et al, 2022).…”

A chromosome-level genome for the nudibranch gastropodBerghia stephanieaehelps parse clade-specific gene expression in novel and conserved phenotypes

“…We cultured Berghia stephanieae juveniles using the same methods as prior B. stephanieae imaging work [35, 36]. We starved juveniles for five days prior to fixation to decrease autofluorescence from digestive contents.…”

“…Berghia stephanieae (hereafter referred to as Berghia ) is a species of gastropod in the order Nudibranchia, a clade of marine slugs that lose their shell during metamorphosis [33]. This species has been used as a model for the study of both more conserved systems, such as neurodevelopment [34] and reproductive development [35], as well as clade-restricted phenotypes such as the sequestration of cnidarian nematocysts [36, 37] and endosymbiosis [38, 39]. We combined an inferred proteome from Berghia with available genome and transcriptome data from other metazoan species – including mollusks such as cephalopods, bivalves, and other gastropods – to identify clade-specific Berghia genes (i.e., restricted to Mollusca, Gastropoda, Heterobranchia, Nudibranchia, Aeolidina, or Berghia alone).…”

A chromosome-level genome for the nudibranch gastropod Berghia stephanieae helps parse clade-specific gene expression in novel and conserved phenotypes

“…Berghia stephanieae (hereafter referred to as Berghia ) is a species of gastropod in the order Nudibranchia, a clade of marine slugs that lose their shell during metamorphosis [ 33 ]. This species has been used as a model for the study of both more conserved systems, such as neurodevelopment [ 34 ] and reproductive development [ 35 ], as well as clade-restricted phenotypes such as the sequestration of cnidarian nematocysts [ 36 , 37 ] and endosymbiosis [ 38 , 39 ]. We combined an inferred proteome from Berghia with available genome and transcriptome data from other metazoan species—including mollusks such as cephalopods, bivalves, and other gastropods—to identify clade-specific Berghia genes (i.e., restricted to Mollusca, Gastropoda, Heterobranchia, Nudibranchia, Aeolidina, or Berghia alone) (Fig.…”