Patterns and Processes of Diploidization in Land Plants

Most land plants are now known to be ancient polyploids that have rediploidized. Diploidization involves many changes in genome organization that ultimately restore bivalent chromosome pairing, disomic inheritance, and resolve dosage and other issues caused by genome duplication. In this review, we discuss the nature of polyploidy and its impact on chromosome pairing behavior. We also provide an overview of two major and largely independent processes of diploidization: cytological diploidization and genic diploidization/fractionation. Finally, we compare variation in gene fractionation across land plants and highlight the differences in diploidization between plants and animals. Altogether, we demonstrate recent advancements in our understanding of variation in the patterns and processes of diploidization in land plants and provide a road map for future research to unlock the mysteries of diploidization and eukaryotic genome evolution. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

DNA Organization along Pachytene Chromosome Axes and Its Relationship with Crossover Frequencies

During meiosis, the number of crossovers vary in correlation to the length of prophase chromosome axes at the synaptonemal complex stage. It has been proposed that the regular spacing of the DNA loops, along with the close relationship of the recombination complexes and the meiotic axes are at the basis of this covariation. Here, we use a cytogenomic approach to investigate the relationship between the synaptonemal complex length and the DNA content in chicken oocytes during the pachytene stage of the first meiotic prophase. The synaptonemal complex to DNA ratios of specific chromosomes and chromosome segments were compared against the recombination rates obtained by MLH1 focus mapping. The present results show variations in the DNA packing ratios of macro- and microbivalents and also between regions within the same bivalent. Chromosome or chromosome regions with higher crossover rates form comparatively longer synaptonemal complexes than expected based on their DNA content. These observations are compatible with the formation of higher number of shorter DNA loops along meiotic axes in regions with higher recombination levels.

Cytogenetical investigations in colchicine-induced tetraploids of Cyamopsis tetragonoloba L.

Successful induction of autotetraploidy has been achieved in five accessions of Cyamopsis tetragonoloba L. The diploid seedlings of these accessions were treated with different concentrations of aqueous colchicine using the cotton-swab method for 10–18 hours within 2–3 days. The highest percentage of success was recorded when the seedlings were treated with 0.2% colchicine for 10 h within two days. The synthesized plants showed remarkable enhancement in several morphological and floral characters making them more robust and better plants from the food and feed aspect. Cytologically, quadrivalent frequency ranging from 3.18 to 5.45 and univalent frequency ranging from 0.08 to 1.10 were characteristic of the colchicine-induced tetraploids. Among all the associations, bivalent chromosome associations were observed more frequently (2.95 to 6.04). The anaphase I and II disjunction of bivalents/chromosomes was leading more or less regularly and equally to the formation of at least few seeds from some of the synthesized plants. Significant enhancement in morphological traits as revealed in colchicine-induced tetraploid plants having a better food and feed value and normal meiotic behaviour of synthesized autotetraploids leading to a good seed set may ultimately result in the genetic improvement of Cyamopsis tetragonoloba.

The conserved kinase NHK-1 is essential for mitotic progression and unifying acentrosomal meiotic spindles in Drosophila melanogaster

Conventional centrosomes are absent from the spindle in female meiosis in many species, but it is not clear how multiple chromosomes form one shared bipolar spindle without centrosomes. We identified a female sterile mutant in which each bivalent chromosome often forms a separate bipolar metaphase I spindle. Unlike wild type, prophase I chromosomes fail to form a single compact structure within the oocyte nucleus, although the integrity of metaphase I chromosomes appears to be normal. Molecular analysis indicates that the mutant is defective in the conserved kinase nucleosomal histone kinase-1 (NHK-1). Isolation of further alleles and RNA interference in S2 cells demonstrated that NHK-1 is also required for mitotic progression. NHK-1 itself is phosphorylated in mitosis and female meiosis, suggesting that this kinase is part of the regulatory system coordinating progression of mitosis and meiosis.

Analysis of the synaptonemal complex of the nine-banded armadillo, Dasypus novemcinctus

The synaptonemal complex (SC) of three specimens of the nine-banded armadillo (Dasypus novemcinctus) was analyzed. Thirty-two bivalents (2n = 64) were observed, 31 of them being autosomes and one an XY sexual bivalent. Chromosome synapsis processes and nucleolus structure changes were analyzed in zygotene and pachytene cells, allowing a detailed description of the beginning of meiotic prophase in this species. There was complete synapsis of X and Y chromosomes. Some abnormalities in SC were observed in cells during zygotene and at the beginning of pachytene, but not in cells in the middle and late pachytene, suggesting the occurrence of synaptic adjustments in their SC.

Cytoembryology of Paspalum chaseanum and Sexual Diploid Biotypes of Two Apomictic Paspalum Species

This study was undertaken to determine the cytology, method of reproduction, and flowering behaviour of three Paspalum species. Paspalum plicatulum has long been considered a segmental allotetraploid that reproduces by obligate apomixis with the type being apospory followed by pseudogamy. Paspalum simplex is an apomictic autotetraploid species, while P. chaseanum is a rare species for which no information regarding cytology and reproduction is available. This investigation concerns diploid cytotypes (2n = 2x = 20) of P. plicatulum and P. simplex that were recently collected in subtropical South America. In addition, two accessions of P. chaseanum were also recorded and both had 2n = 2x = 20 chromosomes. Meiosis showed regular bivalent chromosome pairing. Embryological observations indicated that the three species reproduce sexually at the diploid level. Pollen–pistil interaction following self-pollination suggested the presence of a self-incompatibility system responsible for allogamy. The results indicate that P. plicatulum and P. simplex complexes consist of diploid sexual allogamous cytotypes in addition to the known tetraploid apomictic cytotypes. Diploid strains constitute a source of sexuality for plant improvement. Chromosome doubling will likely produce sexual tetraploids to be used as females in crosses with natural apomictic tetraploid biotypes. Since diploid self-incompatible sexual Paspalum plants usually have apomictic tetraploid co-specific counterparts, the self-incompatible diploid P. chaseanum described in this study warrants further exploration for its apomictic counterpart.

Potential of trispecies bridge crosses and random amplified polymorphic DNA markers for introgression of Medicago daghestanica and M. pironae germplasm into alfalfa (M. sativa)

This report describes the production and cytology of the first interspecific hybrids between cultivated alfalfa (Medicago sativa L.) at the diploid level (2n = 2x = 16) and the diploid (2n = 2x = 16) perennial species M. daghestanica and M. pironae. An ovule–embryo culture technique was required to rescue hybrid embryos and all hybrids were diploid. Predominately bivalent chromosome pairing was observed at meiotic metaphase. All F1 hybrids were male and female sterile and no species backcross progeny could be produced. We discovered that trispecies hybrids could be efficiently recovered via crossing diploid F1 interspecific hybrids of M. sativa × M. rupestris with either M. daghestanica or M. pironae. Ovule–embryo culture was also required to recover these trispecies hybrids with recovery efficiency of trispecies hybrids about 10 times greater than for bispecies hybrids. Most chromosomes paired as bivalents in the trispecies hybrids. Importantly, progeny can be recovered from crossing the trispecies hybrids with M. sativa. Therefore, the M. sativa × M. rupestris hybrids provide a bridge cross to potential introgression of M. daghestanica or M. pironae germplasm. Analysis of randomly amplified polymorphic DNA (RAPD) markers in the trispecies hybrids indicates that RAPD markers offer considerable potential for assaying germplasm introgression following complex hybridizations of the type reported here.Key words: randomly amplified polymorphic DNA, Medicago interspecific hybrids, embryo rescue.

Poleward force at the kinetochore in metaphase depends on the number of kinetochore microtubules.

To examine the dependence of poleward force at a kinetochore on the number of kinetochore microtubules (kMTs), we altered the normal balance in the number of microtubules at opposing homologous kinetochores in meiosis I grasshopper spermatocytes at metaphase with a focused laser microbeam. Observations were made with light and electron microscopy. Irradiations that partially damaged one homologous kinetochore caused the bivalent chromosome to shift to a new equilibrium position closer to the pole to which the unirradiated kinetochore was tethered; the greater the dose of irradiation, the farther the chromosome moved. The number of kMTs on the irradiated kinetochore decreased with severity of irradiation, while the number of kMTs on the unirradiated kinetochore remained constant and independent of chromosome-to-pole distance. Assuming a balance of forces on the chromosome at congression equilibrium, our results demonstrate that the net poleward force on a chromosome depends on the number of kMTs and the distance from the pole. In contrast, the velocity of chromosome movement showed little dependence on the number of kMTs. Possible mechanisms which explain the relationship between the poleward force at a kinetochore, the number of kinetochore microtubules, and the lengths of the kinetochore fibers at congression equilibrium include a "traction fiber model" in which poleward force producers are distributed along the length of the kinetochore fibers, or a "kinetochore motor-polar ejection model" in which force producers located at or near the kinetochore pull the chromosomes poleward along the kMTs and against an ejection force that is produced by the polar microtubule array and increases in strength toward the pole.