Egg-laying Animals Research Articles

Coordination of Reproductive Activity in Aplysia: Peptide Neurohormones, Neurotransmitters, and Pheromones Encoded by the Egg-Laying Hormone Family of Genes.

Pheromones play a significant role in coordinating reproductive activity in the marine opisthobranch mollusk Aplysia. Although solitary during most of the year, these simultaneous hermaphrodites gather into breeding aggregations during the reproductive season. The aggregations contain both mating and egg-laying animals, and are associated with masses of recently deposited egg cordons. Behavioral studies suggest that cordon-derived pheromonal factors are primarily responsible for establishing and maintaining the aggregations. Egg-laying animals are more attractive than sexually mature, but nonlaying, conspecifics and have a shorter mean latency to mating; egg cordons and egg-cordon eluates, when placed in the surrounding seawater, enhance the attractiveness of nonlaying animals and reduce their mean latency to mating. Similar effects are observed when extracts of the atrial gland are placed in the seawater, suggesting that secretory products of this oviductal exocrine organ may function as sexual pheromones. Biochemical analyses indicate that there may be multiple attractants in atrial gland extracts, and that at least one of these (A-NTP) is a peptide encoded by the A gene. The A gene belongs to a small family of structurally related genes that are expressed in a tissue-specific manner. Another member of the family, the egg-laying hormone (ELH) gene, is expressed in the neuroendocrine bag cells. Peptide products of the ELH gene act as neurohormones and nonsynaptic neurotransmitters, initiating egg laying and coordinating its associated behaviors. Peptide products of a family of genes may thus act internally and externally to coordinate both male and female reproductive activities.

Relative Contributions of the Egg Layer and Egg Cordon to Pheromonal Attraction and the Induction of Mating and Egg-laying Behavior in Aplysia.

Many species of the opisthobranch mollusk Aplysia form breeding aggregations during the reproductive season. The aggregations contain both mating and egg-laying animals, and are associated with egg cordons. Although pheromones play a significant role in developing and maintaining the aggregations, little is known about the active factors. Behavioral studies have shown that egg-laying animals are more attractive than nonlaying animals, have shorter latencies to mating, and induce conspecifics to lay eggs. As a first step toward isolating and chemically characterizing the active factors, we examined the relative importance of the egg layer and egg cordon as sources of pheromonal activity in Aplysia brasiliana. T-maze experiments showed that both animal-derived and cordon-derived factors are attractive, and that the animal-derived factors are specifically associated with egg layers. Extracts of the atrial gland--an exocrine organ secreting into the oviduct--increased the attractiveness of nonlaying animals when placed in the surrounding seawater, suggesting that the "cordon-derived" aggregation pheromones may be products of the atrial gland. Mating studies showed that both animal-derived and cordon-derived factors induce mating, and that the animal-derived factors are associated with both egg layers and nonlayers. In contrast, neither animal-derived nor cordon-derived factors induced egg laying. Comparable results were obtained with either one or two animals in the chamber, suggesting that the accessibility of a potential mate did not influence the results. The lack of effect may result from the low-probability nature of egg-laying activity.

Regulation of oogenesis: The piscine receptor for vitellogenin

The receptor-mediated uptake of vitellogenin (VTG), a plasmatic lipophosphoglycoprotein, is crucial for oocyte growth in egg-laying animals. The plasma membrane receptor for VTG was characterized from oocytes of coho salmon, Oncorhynchus kisutch. In direct binding studies, the receptor exhibited high affinity ( K d , 180 nM) for salmonid VTG, and by ligand blotting with radiolabelled VTG it was visualized as a protein with an apparent M r of 100000, under non-reducing conditions. The fish VTG receptor was shown to share key structural elements with VTG receptors from chicken and Xenopus laevis. Namely, cross-reactivity at the level of ligand recognition was observed among VTG receptors from these species and immunological relatedness was demonstrated by immunoblotting with anti-chicken VTG receptor antibodies. In addition, as in chicken and Xenopus, binding of VTG to fish oocyte receptors was shown to be mediated by the lipovitellin domain of VTG. These results clearly indicate that regulation of oocyte growth at the level of yolk formation has been accomplished by the conservation of structural features of receptors required for internalization of VTG.

A new form of reproductive parasitism in cliff swallows

A common reproductive strategy among some egg-laying animals, especially birds, is to lay an egg in another individual's nest and thereby parasitize the reproductive effort of others of either the same species or a different species. Intraspecific parasitism is now known to occur regularly in some species 1–4 and sporadically in many others5,6, and may represent a strategy by which individuals augment their reproductive performance2 or succeed in reproducing when it would otherwise be impossible or too costly5. We have discovered that colonial cliff swallows (Hirundo pyrrhonota) not only lay eggs in other individuals' nests, but also physically transfer eggs between nests after the eggs are laid. Egg transfers can occur at any time after an egg is laid and before it hatches, and may represent a strategy by which an individual distributes its eggs in several nests to ensure some surviving offspring in the event of nesting failures. Sneaky transfer of eggs between nests represents a previously unknown form of reproductive parasitism in birds.