Complexity of the Male Perfume of Eulaema nigrita from Forest and Woody Physiognomies of the Brazilian Savanna: Is There a Relationship with Body Size and Wing Wear?

Male orchid bees collect volatile and semi-volatile compounds from the environment for storage and accumulation in specialized hind legs. Later, these compounds form a perfume blend used during courtship to lure conspecific females for mating. Male orchid bees perfume has been suggested to play an important role as a sexual signaling trait involved in pre-mating isolation of species, functioning as an indicator of male genetic quality. Eulaema nigrita Lepeletier (Apidae: Euglossini) is a common species in both forested and woody savanna (Cerrado stricto sensu) physiognomies of the Brazilian savanna biome. By identifying the chemical composition of male E. nigrita perfume, we tested for differences in the bouquet chemical profile in populations from remnants of seasonal semideciduous forest and woody savanna. In addition, we assessed the relation between perfume complexity and morphological traits associated with size and age of males. Our analysis showed a low effect of physiognomies on differences in the perfume chemical profile of sampled males. Nevertheless, we observed significant differences in the chemical profile of individuals from two seasonal semideciduous forest remnants, which suggests an environmental effect in individual bouquet. Wing wear measurements were positively related to perfume complexity, consistent with the premise that perfumes from older individuals are indicators of survival capacity in male orchid bees.

Beyond tradition: exploring the non-canonical functions of telomeres in meiosis

The telomere bouquet is a specific chromosomal configuration that forms during meiosis at the zygotene stage, when telomeres cluster together at the nuclear envelope. This clustering allows cytoskeleton-induced movements to be transmitted to the chromosomes, thereby facilitating homologous chromosome search and pairing. However, loss of the bouquet results in more severe meiotic defects than can be attributed solely to recombination problems, suggesting that the bouquet's full function remains elusive. Despite its transient nature and the challenges in performing in vivo analyses, information is emerging that points to a remarkable suite of non-canonical functions carried out by the bouquet. Here, we describe how new approaches in quantitative cell biology can contribute to establishing the molecular basis of the full function and plasticity of the bouquet, and thus generate a comprehensive picture of the telomeric control of meiosis.

Meiotic Chromosome Dynamics in Zebrafish

Recent studies in zebrafish have revealed key features of meiotic chromosome dynamics, including clustering of telomeres in the bouquet configuration, biogenesis of chromosome axis structures, and the assembly and disassembly of the synaptonemal complex that aligns homologs end-to-end. The telomere bouquet stage is especially pronounced in zebrafish meiosis and sub-telomeric regions play key roles in mediating pairing and homologous recombination. In this review, we discuss the temporal progression of these events in meiosis prophase I and highlight the roles of proteins associated with meiotic chromosome architecture in homologous recombination. Finally, we discuss the interplay between meiotic mutants and gonadal sex differentiation and future research directions to study meiosis in living cells, including cell culture.

MLKS2 is an ARM domain and F-actin-associated KASH protein that functions in stomatal complex development and meiotic chromosome segregation

The linker of nucleoskeleton and cytoskeleton (LINC) complex is an essential multi-protein structure spanning the eukaryotic nuclear envelope. The LINC complex functions to maintain nuclear architecture, positioning, and mobility, along with specialized functions in meiotic prophase and chromosome segregation. Members of the LINC complex were recently identified in maize, an important scientific and agricultural grass species. Here we characterized Maize LINC KASH AtSINE-like2, MLKS2, which encodes a highly conserved SINE-group plant KASH protein with characteristic N-terminal armadillo repeats (ARM). Using a heterologous expression system, we showed that actively expressed GFP-MLKS2 is targeted to the nuclear periphery and colocalizes with F-actin and the endoplasmic reticulum, but not microtubules in the cell cortex. Expression of GFP-MLKS2, but not GFP-MLKS2ΔARM, resulted in nuclear anchoring. Genetic analysis of transposon-insertion mutations, mlks2-1 and mlks2-2, showed that the mutant phenotypes were pleiotropic, affecting root hair nuclear morphology, stomatal complex development, multiple aspects of meiosis, and pollen viability. In male meiosis, the mutants showed defects for bouquet-stage telomere clustering, nuclear repositioning, perinuclear actin accumulation, dispersal of late prophase bivalents, and meiotic chromosome segregation. These findings support a model in which the nucleus is connected to cytoskeletal F-actin through the ARM-domain, predicted alpha solenoid structure of MLKS2. Functional conservation of MLKS2 was demonstrated through genetic rescue of the misshapen nuclear phenotype of an Arabidopsis (triple-WIP) KASH mutant. This study establishes a role for the SINE-type KASH proteins in affecting the dynamic nuclear phenomena required for normal plant growth and fertility.

Abbreviations: FRAP: Fluorescence recovery after photobleaching; DPI: Days post infiltration; OD: Optical density; MLKS2: Maize LINC KASH AtSINE-like2; LINC: Linker of nucleoskeleton and cytoskeleton; NE: Nuclear envelope; INM: Inner nuclear membrane; ONM: Outer nuclear membrane

The telomere bouquet facilitates meiotic prophase progression and exit in fission yeast

During meiotic prophase, chromosome arrangement and oscillation promote the pairing of homologous chromosomes for meiotic recombination. This dramatic movement involves clustering of telomeres at the nuclear membrane to form the so-called telomere bouquet. In fission yeast, the telomere bouquet is formed near the spindle pole body (SPB), which is the microtubule organising centre, functionally equivalent to the metazoan centrosome. Disruption of bouquet configuration impedes homologous chromosome pairing, meiotic recombination and spindle formation. Here, we demonstrate that the bouquet is maintained throughout meiotic prophase and promotes timely prophase exit in fission yeast. Persistent DNA damages, induced during meiotic recombination, activate the Rad3 and Chk1 DNA damage checkpoint kinases and extend the bouquet stage beyond the chromosome oscillation period. The auxin-inducible degron system demonstrated that premature termination of the bouquet stage leads to severe extension of prophase and consequently spindle formation defects. However, this delayed exit from meiotic prophase was not caused by residual DNA damage. Rather, loss of chromosome contact with the SPB caused delayed accumulation of CDK1-cyclin B at the SPB, which correlated with impaired SPB separation. In the absence of the bouquet, CDK1-cyclin B localised near the telomeres but not at the SPB at the later stage of meiotic prophase. Thus, bouquet configuration is maintained throughout meiotic prophase, by which this spatial organisation may facilitate local and timely activation of CDK1 near the SPB. Our findings illustrate that chromosome contact with the nuclear membrane synchronises meiotic progression of the nucleoplasmic chromosomes with that of the cytoplasmic SPB.

Meiosis-specific cohesin mediates homolog recognition in mouse spermatocytes

During meiosis, homologous chromosome (homolog) pairing is promoted by several layers of regulation that include dynamic chromosome movement and meiotic recombination. However, the way in which homologs recognize each other remains a fundamental issue in chromosome biology. Here, we show that homolog recognition or association initiates upon entry into meiotic prophase before axis assembly and double-strand break (DSB) formation. This homolog association develops into tight pairing only during or after axis formation. Intriguingly, the ability to recognize homologs is retained in Sun1 knockout spermatocytes, in which telomere-directed chromosome movement is abolished, and this is the case even in Spo11 knockout spermatocytes, in which DSB-dependent DNA homology search is absent. Disruption of meiosis-specific cohesin RAD21L precludes the initial association of homologs as well as the subsequent pairing in spermatocytes. These findings suggest the intriguing possibility that homolog recognition is achieved primarily by searching for homology in the chromosome architecture as defined by meiosis-specific cohesin rather than in the DNA sequence itself.

A dynamic meiotic SUN belt includes the zygotene-stage telomere bouquet and is disrupted in chromosome segregation mutants of maize (Zea mays L.)

The nuclear envelope (NE) plays an essential role in meiotic telomere behavior and links the cytoplasm and nucleoplasm during homologous chromosome pairing and recombination in many eukaryotic species. Resident NE proteins including SUN (Sad-1/UNC-84) and KASH (Klarsicht/ANC-1/Syne-homology) domain proteins are known to interact forming the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex that connects chromatin to the cytoskeleton. To investigate the possible cross-kingdom conservation of SUN protein functions in plant meiosis, we immunolocalized maize SUN2 using 3D microscopy of pollen mother cells from maize (Zea mays L.), a large-genome plant model with a canonical NE zygotene-stage telomere bouquet. We detected SUN2 at the nuclear periphery and found that it exhibited a distinct belt-like structure that transitioned to a half-belt during the zygotene stage and back to a full belt during and beyond the pachytene stage. The zygotene-stage half-belt SUN structure was shown by 3D immuno-FISH to include the NE-associated telomere cluster that defines the bouquet stage and coincides with homologous chromosome synapsis. Microtubule and filamentous actin staining patterns did not show any obvious belt or a retracted-like structure other than a general enrichment of tubulin staining distributed widely around the nucleus and throughout the cytoplasm. Genetic disruption of the meiotic SUN belt staining patterns with three different meiosis-specific mutants, desynaptic (dy1), asynaptic1 (as1), and divergent spindle1 (dv1) provides additional evidence for the role of the nuclear envelope in meiotic chromosome behavior. Taking into account all of the observations from this study, we propose that the maize SUN belt is directly or indirectly involved in meiotic telomere dynamics, chromosome synapsis, and possibly integration of signals and forces across the meiotic prophase nuclear envelope.

DNA double-strand breaks, but not crossovers, are required for the reorganization of meiotic nuclei in Tetrahymena

During meiosis, the micronuclei of the ciliated protist Tetrahymena thermophila elongate dramatically. Within these elongated nuclei, chromosomes are arranged in a bouquet-like fashion and homologous pairing and recombination takes place. We studied meiotic chromosome behavior in Tetrahymena in the absence of two genes, SPO11 and a homolog of HOP2 (HOP2A), which have conserved roles in the formation of meiotic DNA double-strand breaks (DSBs) and their repair, respectively. Single-knockout mutants for each gene display only a moderate reduction in chromosome pairing, but show a complete failure to form chiasmata and exhibit chromosome missegregation. The lack of SPO11 prevents the elongation of meiotic nuclei, but it is restored by the artificial induction of DSBs. In the hop2ADelta mutant, the transient appearance of gamma-H2A.X and Rad51p signals indicates the formation and efficient repair of DSBs; but this repair does not occur by interhomolog crossing over. In the absence of HOP2A, the nuclei are elongated, meaning that DSBs but not their conversion to crossovers are required for the development of this meiosis-specific morphology. In addition, by in silico homology searches, we compiled a list of likely Tetrahymena meiotic proteins as the basis for further studies of the unusual synaptonemal complex-less meiosis in this phylogenetically remote model organism.