Insights into in vitro spermatogenesis in mammals: Past,present, future

Considering the self‐renewal and differentiation ability of pluripotent stem cells, some studies have pointed out the possibility of stem cell‐derived sperm production. Most studies that test this hypothesis have been conducted on rodents, with some promising results; however, studies on humans are progressing slowly, and have encountered technical and ethical hurdles. Established methods to differentiate stem cells—including embryoid bodies, co‐culturing, and various feeder cells—may provide a niche that is similar to in vivo conditions and resolve epigenetic abnormalities, but a gonadal‐like three‐dimensional structure is still required to produce germ cells with the correct imprinting. In the last few years, sperm‐like cells with fertilizing capacity were produced from mouse embryonic stem cells, and the resulting embryos from these cells yielded live offspring. Future research should move towards the use of adult stem cells, however, owing to the unavailability of embryonic cells in adults. More intensive research and techniques are required since in vitro spermatogenesis provides hope to individuals without mature sperm who cannot be treated, and may be a useful system to study the precise mechanism of spermatogenesis. In this review, we describe recent studies of in vitro spermatogenesis mechanisms and related techniques in mammals. We also discuss the possible cell surface markers and culture conditions that might improve in vitro spermatogenesis.     

Subcellular preservation in giant ostracod sperm from an early Miocene cave deposit in Australia

Cypridoidean ostracods are one of a number of animal taxa that reproduce with giant sperm, up to 10 000 µm in length, but they are the only group to have aflagellate, filamentous giant sperm. The evolution and function of this highly unusual feature of reproduction with giant sperm are currently unknown. The hypothesis of long-term evolutionary persistence of this kind of reproduction has never been tested. We here report giant sperm discovered by propagation phase contrast X-ray synchrotron micro- and nanotomography, preserved in five Miocene ostracod specimens from Queensland, Australia. The specimens belong to the species Heterocypris collaris Matzke-Karasz et al. 2013 (one male and three females) and Newnhamia mckenziana Matzke-Karasz et al. 2013 (one female). The sperm are not only the oldest petrified gametes on record, but include three-dimensional subcellular preservation. We provide direct evidence that giant sperm have been a feature of this taxon for at least 16 Myr and provide an additional criterion (i.e. longevity) to test hypotheses relating to origin and function of giant sperm in the animal kingdom. We further argue that the highly resistant, most probably chitinous coats of giant ostracod sperm may play a role in delaying decay processes, favouring early mineralization of soft tissue.     

Sphondylothamnion multifidum (Huds.) Näg. in western Europe

The lipid composition of 140 clonal isolates of brown algae, covering 16 species of the genera Ectocarpus, Feldmannia and Hincksia, was analysed. All taxa contained the glycolipids monogalactosyldiacylglycerol, digalactosyldiacylglycerol and sulphoquinovosyldiacylglycerol, and the phospholipids phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and an unidentified PX. Presence of the betaine lipid diacylglycerylhydroxymethyltrimethyl-β-alanine (DGTA) was strongly correlated with certain species in the genera Hincksia and Ectocarpus. While seven species of Hincksia contained DGTA, this lipid was absent in H. granulosa and in an unidentified Hincksia species. Due to heteromorphy of generations, species assignments in the genus Ectocarpus were made only for isolates for which the complete life history was known. We found that E. fasciculatus contains DGTA, while this lipid is absent in E. siliculosus. A collection of 40 non-sexual Ectocarpus strains included representatives with and without DGTA. The value of DGTA as a taxonomic marker in Ectocarpus and Hincksia is discussed.     

Cytological,morphological and molecular analyses of controlled progenies from meiotic mutants of alfalfa producing unreduced gametes

A program of sexual polyploidization was carried out in alfalfa using plants from wild diploid species that produced male or female unreduced gametes. Sixteen progenies from 2x-4x and 2x-2x crosses were examined with a combination of morphological, cytological and molecular analyses. The chromosome counts revealed diploid, tetraploid and aneuploid plants. Plants with B chromosomes were also detected. The leaf area of the plants was a useful characteristic for distinguishing tetraploid from diploid plants obtained by unilateral or bilateral sexual polyploidization. Leaf shape and leaf margin were not correlated with the ploidy levels. Plants with supernumerary chromosomes displayed obovate or elliptic leaves which differed markedly from the range of forms typical of diploid and tetraploid alfalfa plants. RAPD markers were investigated in all progeny plants to determine maternal and paternal amplification products. Three alfalfa-specific primers proved to be effective in revealing the hybrid origin of the plants. A combination of cytological, morphological and molecular analyses is essential for a detailed genetic characterization of progenies in programs of sexual polyploidization.     

Gamete signalling underlies the evolution of mating types and their number

The gametes of unicellular eukaryotes are morphologically identical, but are nonetheless divided into distinct mating types. The number of mating types varies enormously and can reach several thousand, yet most species have only two. Why do morphologically identical gametes need to be differentiated into self-incompatible mating types, and why is two the most common number of mating types? In this work, we explore a neglected hypothesis that there is a need for asymmetric signalling interactions between mating partners. Our review shows that isogamous gametes always interact asymmetrically throughout sex and argue that this asymmetry is favoured because it enhances the efficiency of the mating process. We further develop a simple mathematical model that allows us to study the evolution of the number of mating types based on the strength of signalling interactions between gametes. Novel mating types have an advantage as they are compatible with all others and rarely meet their own type. But if existing mating types coevolve to have strong mutual interactions, this restricts the spread of novel types. Similarly, coevolution is likely to drive out less attractive mating types. These countervailing forces specify the number of mating types that are evolutionarily stable.This article is part of the themed issue ‘Weird sex: the underappreciated diversity of sexual reproduction’.     

Postovulatory aging of human ova: II. Spontaneous division of the first polar body

Two oocytes recovered from the Fallopian tube at 24 and 50 h following the LH peak in plasma were examined by light microscopy and transmission electron microscopy. The possibility of fertilization was excluded by clinical and morphologic evidence. In the earliest specimen, the first polar body showed condensed nuclear chromatin, without nuclear envelope, and is interpreted as entering interphase. In the other specimen, the first polar body was undergoing division, which is interpreted as spontaneous second meiotic division.     

Estimation of preferential pairing in tetraploid x diploid hybridizations

Summary Crosses made between tetraploid and diploid, 2n pollen-producing species directly transfer from one-half to the entire diploid genome from the diploid to the tetraploid level, depending on the mechanism of 2n pollen formation and the amount of crossing-over that occurs. Tetraploid plants that result from tetraploid x diploid hybridizations can be further utilized in a breeding program. It is postulated that preferential pairing between homologous chromosomes derived from the original tetraploid or diploid parent occurs in the tetraploid x diploid hybrid. Depending on the genetic divergence of the species involved, preferential pairing of homologous chromosomes may range from zero to one. Theoretical estimates of the amount of preferential pairing and the standard errors of these estimates are derived for cases where the diploid parent produces 2n gametes by either a first division or a second division restitution mechanism.