Crossovers (COs) shuffle genetic information and allow balanced segregation of homologous chromosomes during the first division of meiosis. In several organisms, mutants demonstrate that two molecularly distinct pathways produce COs. One pathway produces class I COs that exhibit interference (lowered probability of nearby COs), and the other pathway produces class II COs with little or no interference. However, the relative contributions, genomic distributions, and interactions of these two pathways are essentially unknown in nonmutant organisms because marker segregation only indicates that a CO has occurred, not its class type. Here, we combine the efficiency of light microscopy for revealing cellular functions using fluorescent probes with the high resolution of electron microscopy to localize and characterize COs in the same sample of meiotic pachytene chromosomes from wild-type tomato. To our knowledge, for the first time, every CO along each chromosome can be identified by class to unveil specific characteristics of each pathway. We find that class I and II COs have different recombination profiles along chromosomes. In particular, class II COs, which represent about 18% of all COs, exhibit no interference and are disproportionately represented in pericentric heterochromatin, a feature potentially exploitable in plant breeding. Finally, our results demonstrate that the two pathways are not independent because there is interference between class I and II COs.E ukaryotic sexual reproduction involves meiosis, a specialized cell division in which DNA duplication in a diploid cell is followed by two cell divisions to produce four haploid cells. The first division, Meiosis I, involves crossing over and chiasmata formation between each pair of homologous chromosomes, thereby ensuring separation of the homologs and formation of two haploid cells, each with one complete set of replicated chromosomes. The second division, Meiosis II, is a mitosis-like division in which the two sister chromatids separate to yield four haploid cells that directly or indirectly form gametes. Because these four products are genetically unique due to crossing over and independent segregation of homologous chromosomes during Meiosis I, meiosis plays an important role in creating genetic diversity in sexually reproducing organisms.Crossing over during meiosis is tightly controlled so each pair of homologs has at least one "obligate" crossover (CO) that ensures balanced reductional segregation, but the presence of a CO reduces the likelihood of another CO in its vicinity, a phenomenon referred to as CO interference (1, 2). Significant progress has been made recently in illuminating the molecular events of meiotic recombination and the control of crossing over (3)(4)(5)(6)(7)(8). The initiating event of meiotic recombination in most organisms is formation of numerous DNA double-strand breaks (DSBs). Homolog-dependent repair of a DSB may follow any one of at least three pathways: (i) non-CO that may result in a short gene conversion; (ii) CO ...

We monitored temperatures during the winter of 1995–1996 inside 18 nests containing hatchling painted turtles (Chrysemys picta). The study was performed at the Valentine National Wildlife Refuge in north-central Nebraska to assess survival of neonatal turtles in relation to the thermal environment inside their hibernacula. Minimum temperatures in the nests varied from −3 to −21 °C, and were better predictors of survival of hatchlings than other measures of the thermal environment. All hatchlings survived in nests where the temperature never went below −7 °C, some animals survived in nests where the minimum was between −7 and −13 °C, but no turtle survived in a nest where the minimum was below −14 °C. Hatchlings probably survived the cold by sustaining a state of supercooling, because the duration of exposure to low temperatures was far too long for animals in most nests to have survived in a frozen state.

We studied tolerance for cold in hatchling painted turtles (Chrysemys picta) from Lake Metigoshe, Bottineau County, North Dakota, to determine whether neonates in populations near the northern limit of distribution rely on a tolerance for freezing or on a capacity for supercooling to survive their first winter of life. We placed hatchlings individually into artificial hibernacula constructed in jars of damp, loamy sand and then cooled the jars to approximately -0.45 degrees C, which was below the equilibrium freezing point for water held by the sand but above that for body fluids of the neonatal turtles. A piece of ice next was placed on the surface of the sand in each jar to induce freezing of the soil water. After the soil water had frozen to an equilibrium, the temperature in the jars was lowered by 1 degrees C/d to minima averaging -2.5 degrees C, -4.5 degrees C, -6.5 degrees C, and -10.5 degrees C in different treatments. These temperatures were maintained for varying periods, so that animals in each treatment were exposed to temperatures below the equilibrium freezing point for their body fluids for a total of 11 d. Thirty of 32 hatchlings survived exposure to -2.5 degrees C; 24 of 32 survived at -4.5 degrees C; 14 of 32 withstood -6.5 degrees C; and 7 of 32 tolerated -10.5 degrees C. Freezing exotherms were detected in temperature profiles for turtles that succumbed but not in those for hatchlings that survived. Thus, the ability of hatchlings to withstand subzero temperatures for extended periods apparently requires that they avoid freezing. Although other workers contend that tolerance for freezing is the key to survival over winter by hatchling painted turtles from the region of Lake Metigoshe, our findings indicate that neonates rely primarily on their ability to remain unfrozen and supercooled.

Early recombination nodules (ENs) are multiprotein complexes that are thought to be involved in synapsis and recombination, but little is known about their components or how they may be involved in these events. In this study, we describe the cytological behavior of a possible EN component, MRE11, a protein that is important for the repair of the numerous, programmed deoxyribonucleic acid double-strand breaks (DSBs) that occur early in the meiotic prophase. By immunofluorescence, many MRE11 foci were associated with chromosomal axes during early prophase I in both wild-type Arabidopsis and tomato primary microsporocytes. Similar patterns of MRE11 foci were observed in two Arabidopsis mutants (Atspo11-1 and Atprd1) that are defective in DSB formation and synapsis. In tomato chromosomes, MRE11 foci were more common in distal euchromatin than in proximal heterochromatin, consistent with known EN patterns. However, electron microscopic immunogold localization demonstrated that only about 10% of ENs were labeled, and most MRE11 label was associated with synaptonemal complex components. Thus, in plants, MRE11 foci are not dependent on DSB formation, and most MRE11 foci do not correspond to ENs. More generally, our results show that the simple presence of large numbers of fluorescent foci associated with synapsing chromosomes is insufficient evidence to equate these foci with ENs.

Proteins of the cohesin complex are essential for sister chromatid cohesion and proper chromosome segregation during both mitosis and meiosis. Cohesin proteins are also components of axial elements/lateral elements (AE/LEs) of synaptonemal complexes (SCs) during meiosis, and cohesins are thought to play an important role in meiotic chromosome morphogenesis and recombination. Here, we have examined the cytological behavior of four cohesin proteins (SMC1, SMC3, SCC3, and REC8/SYN1) during early prophase I in tomato microsporocytes using immunolabeling. All four cohesins are discontinuously distributed along the length of AE/LEs from leptotene through early diplotene. Based on current models for the cohesin complex, the four cohesin proteins should be present at the same time and place in equivalent amounts. However, we observed that cohesins often do not colocalize at the same AE/LE positions, and cohesins differ in when they load onto and dissociate from AE/LEs of early prophase I chromosomes. Cohesin labeling of LEs from pachytene nuclei is similar through euchromatin, pericentric heterochromatin, and kinetochores but is distinctly reduced through the nucleolar organizer region of chromosome 2. These results indicate that the four cohesin proteins may form different complexes and/or perform additional functions during meiosis in plants, which are distinct from their essential function in sister chromatid cohesion.