Hypothermic storage in salt-free preservative solution alter motility duration in sterlet sperm

Together with concentration, motility is one of the most important characteristics of sturgeon sperm, determining its quality and suitability for insemination. After activation in water, the duration of progressive sperm motility is also important, and this time should not be less than that required for fertilization. Motility of spermatozoa depends on their physiological state, maturity, age and intracellular reserves of macroergic substances. During hypothermic storage, the percentage of spermatozoa that can be activated decreases progressively due to depletion of ATP supply or cell death. To improve the hypothermic storage of sterlet sperm, we have developed salt-free preservative solution ISGT-80 based on glucose and trehalose. During storage of sterlet sperm specimens from 20 males in ISGT-80 for 18 days, we observed, along with a progressive decrease in the percentage of motile spermatozoa, an alteration in the duration of their motility. On the 3rd to 6th day of storage, the time of half-loss of motility (τ50) increased significantly by approximately 1 min on average compared with fresh samples, then gradually decreased, however, not descending to the initial value. The reasons for this prolongation of motility are not clear, but we do not exclude the predominant death of spermatozoa with a short motility duration in the first days of storage and selection in favor of long-lived spermatozoa. Such gametic selection can lead to a shift in allele frequencies at heterozygous loci in the offspring. Thus, hypothermic storage of sperm could become an attractive subject for genetic research with the aim of developing new selection tools in sturgeon breeding.

Natural Selection in Large Populations

This chapter includes selection in haploid and diploid organisms, hard and soft selective sweeps, background selection, and the probability of ultimate survival of a new favorable mutation in a large population. It considers overdominance and heterozygote inferiority in detail as well as different types of equilibria and the fundamental theorem of natural selection. Various types of balancing selection are examined including mutation–selection balance, migration–selection balance, meiotic drive and gametic selection, and the theory of CRISPR-mediated gene drive to control natural populations. It closes with a discussion of other modes of selection and their implications.

The role of multiple reproductive barriers: strong post-pollination interactions govern cytotype isolation in a tetraploid–octoploid contact zone

Background and Aims Polyploidy is an important contributor to sympatric speciation and assortative mating is a key mechanism driving cytotype interactions in contact zones. While strong reproductive barriers can mediate the coexistence of different cytotypes in sympatry, positive frequency-dependent mating disadvantage ultimately drives the transition to single-ploidy populations. However, comprehensive estimates of reproductive isolation among cytotypes and across multiple barriers are rare. We quantify the strength of isolation across multiple reproductive stages in a tetraploid–octoploid contact zone to understand the potential for coexistence. Methods Assortative mating due to flowering asynchrony, pollinator behaviour, morphological overlap, self-fertilization and gametic competition between tetraploid and octoploid Gladiolus communis in a contact zone in the Western Iberian Peninsula were assessed in natural and experimental populations to quantify reproductive isolation (RI) between cytotypes. Key Results Tetraploids and octoploids have a high degree of overlap in flowering time and similar floral morphology, and are visited by generalist insects without cytotype foraging preferences, resulting in weak pre-pollination RI (from 0.00 to 0.21). In contrast, post-pollination isolation resulting from gametic selection was a strong barrier to inter-cytotype mating, with ploidy composition in stigmatic pollen loads determining the levels of RI (from 0.54 to 1.00). Between-cytotype cross-incompatibility was relatively high (RI from 0.54 to 0.63) as was isolation acquired through self-pollination (RI of 0.59 in tetraploids and 0.39 in octoploids). Conclusions Total RI was high for both tetraploids (from 0.90 to 1.00) and octoploids (from 0.78 to 0.98). Such high rates of assortative mating will enable cytotype coexistence in mixed-ploidy populations by weakening the impacts of minority cytotype exclusion. This study reveals the key role of gametic selection in cytotype siring success and highlights the importance of comprehensive estimates across multiple reproductive barriers to understand cytotype interactions at contact zones.

A natural, conditional gene drive in plants

A new class of gene drive in plant populations with herbicide resistance is described; a conditional gene drive that operates following herbicide application. Screening progeny from controlled crosses of Brassica napus heterozygous for a dominant allele conferring chlorsulfuron resistance, demonstrated that the herbicide imposes in planta gametic selection against pollen and ovules with the recessive allele for herbicide susceptibility, as well as embryonic selection against embryos homozygous for the susceptible allele. We postulate that natural gene drives are common in plant populations and can operate in a conditional manner resulting in non-Mendelian inheritance in response to abiotic and biotic stresses.

The genetic architecture of recurrent segregation distortion in Arabidopsis thaliana

The equal probability of transmission of alleles from either parent during sexual reproduction is a central tenet of genetics and evolutionary biology. Yet, there are many cases where this rule is violated. Such violations limit intraspecific gene flow and can facilitate the formation of genetic barriers, a first step in speciation. Biased transmission of alleles, or segregation distortion, can result from a number of biological processes including epistatic interactions between incompatible loci, gametic selection, and meiotic drive. Examples of these phenomena have been identified in many species, implying that they are universal, but comprehensive species-wide studies of segregation distortion are lacking. We have performed a species-wide screen for distorted allele frequencies in over 500 segregating populations of Arabidopsis thaliana using reduced-representation genome sequencing. Biased transmission of alleles was evident in up to a quarter of surveyed populations. Most populations exhibited distortion at only one genomic region, with some regions being repeatedly affected in multiple populations. Our results begin to elucidate the species-level architecture of biased transmission of genetic material in A. thaliana, and serve as a springboard for future studies into the basis of intraspecific genetic barriers.

Detection and mapping of a lethal locus in a eucalyptus hybrid population

The objective of this work was to verify the existence of a lethal locus in a eucalyptus hybrid population, and to quantify the segregation distortion in the linkage group 3 of the Eucalyptus genome. A E. grandis x E. urophylla hybrid population, which segregates for rust resistance, was genotyped with 19 microsatellite markers belonging to linkage group 3 of the Eucalyptus genome. To quantify the segregation distortion, maximum likelihood (ML) models, specific to outbreeding populations, were used. These models consider the observed marker genotypes and the lethal locus viability as parameters. The ML solutions were obtained using the expectation‑maximization algorithm. A lethal locus in the linkage group 3 was verified and mapped, with high confidence, between the microssatellites EMBRA 189 e EMBRA 122. This lethal locus causes an intense gametic selection from the male side. Its map position is 25 cM from the locus which controls the rust resistance in this population.

A Chromosomal Region Promoting Outcrossing in a Conifer

Prefertilization mechanisms influencing selfing rates are thought to be absent in conifers. Outcrossing in conifers is promoted via an embryo-lethal system, but the genetic mechanism is poorly understood. This study is the first experimental profile of the genetic mechanism promoting outcrossing in conifers. Molecular dissection of a Pinus taeda L. selfed pedigree detected a chromosomal region identified as PtTX3020-RPtest9. Within this region, a semilethal factor was tightly linked (r = 0.0076) to a polymorphic expressed sequence tag (EST). The linkage group flanking the lethal factor showed strong heterozygote advantage. Using genotypic frequencies for the linkage group, three hypotheses about the semilethal factor could be tested: (1) the presence of a balanced lethal system, i.e., a lethal factor present in each of the two marker intervals; (2) gametic selection operative prior to fertilization; and (3) a stage-specific lethal factor. Selection acted via the embryo-lethal system. No support for a genetic mechanism operating prior to fertilization was found. The semilethal factor exerted no effect after embryo maturity. The genetic mechanism promoting outcrossing in P. taeda L. appears to have a balancing selection system due to either pseudo-overdominance or true overdominance.