Heterotic effect in two generations of experimental crossing of lines of steppe lemming, Lagurus lagurus (Rodentia, Mammalia)

Both in natural environments and during controlled breeding, animals encounter situations involving inbreeding and hybridization of inbred forms. The effects of inbreeding on individual components of fitness, such as viability and fertility, have primarily been studied in economically significant species. However, as we recognize that contrast to existing ideas about the ongoing evolution of the biosphere, degradation of the biosphere is occurring, it is essential to reevaluate the strategies for human interaction with nature. These strategies should be designed to preserve systemic organization and maintain intra- and inter-population diversity at optimal levels. When applied correctly, hybridization can serve both to establish and substantiate the species status of difficult-to-distinguish forms and to enhance valuable traits in economically important breeds while preserving the genetic diversity of domesticated animals, including rare species. To investigate the influence of intraspecific hybridization on the reproductive, behavioral, and morphological characteristics of animals, four successive cycles of inbred breeding (iF1-iF4) of the steppe lemming (Lagurus lagurus) from two distinct habitats were conducted, followed by reciprocal crossing of the resulting stocks in the hF1 and hF2 generations. Under laboratory conditions, two lines of steppe lemmings were established, bred according to a standard brother-sister scheme. The founders of these lines were descendants of animals captured in Novosibirsk Oblast: line K was sourced from the Karasuk district (forest-steppe zone of Northern Kulunda), while line CH was sourced from the Zdvinsk district (steppe areas of the Barabinskaya lowland near Lake Chany). Animals bred within each of the drains, with a random selection of pairs serving as controls, were studied. After being separated from their parents, young animals were housed in same-sex sibling groups. At the age of 1 to 1.5 months, they were individually housed. Animals that were two months old were used for breeding. The female was placed in the male's cage, and the appearance of the cubs was monitored. Each breeding pair was given the opportunity to produce up to two broods. Experimental animals were bred inbred over four successive generations. To obtain first-generation hybrids (hF1), reciprocal crossing of the descendants from two lines of the fourth generation was conducted, resulting in the formation of 32 pairs. To obtain second-generation hybrids (hF2), 20 pairs were formed among the hF1 hybrids. The weight of females, the number of live and stillborn cubs, and the growth and survival of litters until the end of the feeding period were recorded. The day after the birth of the first litter, both the female and male were assessed for parental care for 10 minutes. They were removed from the cage, and the young were relocated from the nest to the opposite corner (approximately 20 cm away). Each parent was then placed sequentially in the nest, and the time until the first pup was returned to the nest was measured. After completing double reproduction (at the age of 3 to 4 months), the animals were removed from the experiment. Body length and weight, as well as the weight of internal organs, were determined posthumously. For calculations, indices were utilized, specifically the ratio of the mass of internal organs to body weight. During inbreeding, the number of live births and the number of cubs raised to independence decreased compared to the control group, primarily due to an increase in juvenile mortality (see Fig. 1). In contrast, when two lines of steppe lemmings were hybridized, fertility increased significantly, as evidenced by a rise in both the number of live births and the number of pups raised to independence compared to those that produced inbred offspring. Body condition indicators were highest in inbred animals, which may be attributed to “favourable” housing conditions (see Fig. 5). However, reproductive rates for both males and females declined. In inbred breeding, the proportion of successfully reproducing pairs decreased significantly compared to the control group. During hybridization, this proportion returned to initial values (see Table 2). Additionally, a decrease in the relative weight of the testes in inbred males was observed, which we attribute, on one hand, to the negative effects of inbreeding and, on the other hand, to the conditions of captivity (see Fig. 4). Parental care did not diminish over four generations of inbreeding; in fact, it increased in the first two generations for both females and males when compared to the control group (see Fig. 3). This enhancement in parental care may have improved the survival chances of offspring from parents with reduced reproductive rates. The article contains 5 Figures, 2 Tables, 39 References. the authors are grateful to the teacher, Prof. Vadim Ivanovich Evsikov, who initiated this research and managed to participate in it during his lifetime, and Gulmira Tolegenovna Kokenova, with whose participation the work with animals was carried out. The Authors declare no conflict of interest.

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