MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene activity in cells and are present in the milk of terrestrial mammals. Recent studies have, for the first time, described this class of molecules in bottlenose dolphins (Tursiops truncatus) and identified among the most common miRNAs several already known families.

Composition of miRNAs in Milk

Milk contains not only proteins, fats, carbohydrates, vitamins, and other nutrients, but also miRNAs. These molecules do not code for proteins, but they can influence gene expression by binding to other RNAs and regulating protein synthesis.

In humans and other terrestrial mammals, the composition of miRNAs in milk has been studied in detail. miRNAs are associated with the development of the immune system, the gut, fat metabolism, and other important processes in early life. For marine mammals, such data were previously almost nonexistent. The new study has filled this gap for bottlenose dolphins.

Genomic Analysis and Species Comparison

During the research, the dolphin genome was analyzed, revealing 186 miRNA families and 354 high-confidence precursors—longer molecules from which mature miRNAs are formed. The dolphin genome was compared with the genomes of 52 other cetacean species to determine which miRNA families have been preserved across different species and which may have been lost during evolution.

Study of Dolphin Milk

To analyze miRNAs in milk, 76 samples were collected from two female dolphins at an Italian marine park at various stages of lactation—from the first to the 14th month after the calf’s birth and from the 14th to the 38th month. All samples were combined into a single pool, from which RNA was extracted and small molecules were sequenced.

Sequencing yielded 45.6 million individual RNA fragments. After removing technical sequences and filtering, 1.54 million reads between 18 and 24 nucleotides in length remained for further analysis. Of these, 119 miRNAs reached the established detection threshold.

Comparison with Other Mammals

In dolphin milk, miRNA families were found that are also present in the milk of humans, cows, goats, donkeys, buffalo, pigs, and sheep. Some of these molecules are involved in processes essential for the development of offspring. For example, mir-148 is linked to fat and lipid metabolism and may also influence immune responses, while mir-101 is involved in regulating genes responsible for cell proliferation and the response to oxidative stress.

Prospects for Further Research

It is still too early to say whether miRNAs in dolphin milk function in the same way as in terrestrial mammals. The similarity has so far been established only qualitatively: the same main miRNA families have been identified in animals from different groups. The next stage of research will involve analyzing a larger number of animals and separate stages of lactation.