Phylogeography of cladocerans in the Northern Palearctic
A group of scientists, including the researchers from the White Sea Biological Station, Lomonosov Moscow State University has studied dispersal routes of cladocerans through Northern Eurasia, which are a food for many fish species. The scientists have shown that at least several cladoceran taxa began colonization of the whole Palearctic from its north, and some of them from the Beringian region. The Bering Strait has closed numerous times in the past; modern-day Kamchatka, Chukotka, Alaska and a part of the Aleutian Islands were once a part of Beringia. The results of this study are published in PLOS ONE.
Determination of the nucleotide sequence in some genes of the cladocerans was undertaken at the White Sea Biological Station, Lomonosov Moscow State University. Tatyana Neretina, Ph.D., a researcher of the biological station and study co-author notes, "It could seem ridiculous to bring samples to the White Sea, to Polar Circle, but our laboratory is equipped so well and it's so convenient to work there that colleagues come to us and work with our collections, as well as with samples from all over the world, including those gathered in Antarctica by the Polarstern research icebreaker. In most cases, we study samples in order to understand what species inhabit these high latitudes. However, in this particular case, we've studied biogeography."
Freshwater invertebrates, including small crustaceans, are a cornerstone of many ecosystems, and their evolution is closely intertwined with the fates of many other species. All these comprise the basis of biodiversity development in the Northern Hemisphere. Cladocerans' eggs could travel from one water body to another within special containers left after molting, which are called ephippiua, and disperse to new inhabiting areas (for instance, on the feathers of some aquatic birds).
Biogeography deals with investigations of dispersal routes of living organisms. According to the authors of the current paper, biogeography of freshwater animals is much less developed, and dispersion patterns in freshwater animals strongly differ from those of terrestrial inhabitants. This is due to differences in the biology and geological age of freshwater and terrestrial animals.
Previous researchers studied only cladocerans belonging to the genus Daphnia. According to these data, Japan was the center of dispersion of several groups of species across Eastern Eurasia. In order to check such conclusions on other groups of the cladocerans, hydrobiologists are now conducting a genetic analysis. A strong progress in this direction has been achieved by Russian scientists by means of a study of the cladocerans from the Chydorus sphaericus group, which are common and most typical inhabitants of the northern regions. To achieve this aim, scientists have investigated the samples of cladocerans from different regions of Russia, including Kronotsky and Komandorsky Nature Reserves, as well as samples from Norway and South Korea.
In order to reveal the centers of dispersion, scientists compare features of descendants and ancestors of a group. Afterwards, they look for taxa, which have more ancestral features as these taxa are regarded as nearest to an initial group. Such comparisons have been made previously based exclusively on morphological features, however, modern scientists understand that it's not enough for adequate conclusions. Tatyana Neretina explains: "Studying only morphological features could say a little about how they evolve and disperse. That's why nowadays every zoologist and botanist needs to know molecular genetic methods."
Routes of cladocerans and zoologists
Comparison of the gene sequences reveals the age of the division of different lines and groups of living organisms. The speed of mutation accumulation varies among different genes. Most important genes are less variable, and vice versa. As a result, scientists apply "molecular clocks" to determine congeniality and age of divergence of various groups of living organisms.
Tatyana Neretina says, "You collect material from different places, preserve it in alcohol, extract DNA, and after that, with the help of polymerase chain reaction (PCR) techniques, determine the order of nucleotides in the genome fragment. Then you look at the similarity between different specimens and construct phylogenetic trees, which help to understand the relationships between organisms."
In case of the Chydorus sphaericus group the researchers have studied two genes: (1) a portion of the mitochondrial gene encoding an important enzyme—cytochrome c; (2) a portion of the nuclear DNA, where two parts of ribosomal RNA (necessary for protein synthesis in a cell) are coded and the indel between them.
Comparisons of gene sequences have helped hydrobiologists to determine relationships between groups and trace pattern of their dispersion across Northern Eurasia. For this purpose, scientist have built median-joining haplotype networks (a haplotype is a group of organisms that all have exactly the same gene sequence) in order to determine regions inhabited by ancestral groups of Chydorus sphaericus to learn where their initial center of dispersion is located. It was found that centers of dispersion of two groups were located in the European part of the continent and two others in Asia. One clade survived during harsh conditions of the Pleistocene glaciation in a northern refugium, while another clade survived in the south.
Tatyana Neretina says, "The concept of a refugium implies the absence of ice there, and it's not so important whether it was southern or northern. No special adaptations are needed in order to survive glaciation inside a refugium. In contrast, our study shows that in refugia, the animals already had all necessary adaptations to be widely dispersed."
Another group of Chydorus sphaericus colonized Bering Island from the continental Beringian region (the former was not connected with the latter in the past) three times independently, not simultaneously. In the future, this scientific team, which also includes researchers from the Severtsov Institute of Ecology and Evolution of Russian Academy of Sciences, Institute for Inlanf Water Biology of Russian Academy of Sciences and the Faculty of Biology of the State University of New York, is going to test applicability of revealed scheme flexibility to other cladoceran genera.