MLH1-focus mapping in birds shows equal recombination between sexes and diversity of crossover patterns

Meiosis through three centuries

Meiosis is the specialized cellular program that underlies gamete formation for sexual reproduction. It is therefore not only interesting but also a fundamentally important subject for investigation. An especially attractive feature of this program is that many of the processes of special interest involve organized chromosomes, thus providing the possibility to see chromosomes "in action". Analysis of meiosis has also proven to be useful in discovering and understanding processes that are universal to all chromosomal programs. Here we provide an overview of the different historical moments when the gap between observation and understanding of mechanisms and/or roles for the new discovered molecules was bridged. This review reflects also the synergy of thinking and discussion among our three laboratories during the past several decades.

Fine-Scale Map Reveals Highly Variable Recombination Rates Associated with Genomic Features in the Eurasian Blackcap

Recombination is responsible for breaking up haplotypes, influencing genetic variability, and the efficacy of selection. Bird genomes lack the protein PR domain-containing protein 9, a key determinant of recombination dynamics in most metazoans. Historical recombination maps in birds show an apparent stasis in positioning recombination events. This highly conserved recombination pattern over long timescales may constrain the evolution of recombination in birds. At the same time, extensive variation in recombination rate is observed across the genome and between different species of birds. Here, we characterize the fine-scale historical recombination map of an iconic migratory songbird, the Eurasian blackcap (Sylvia atricapilla), using a linkage disequilibrium-based approach that accounts for population demography. Our results reveal variable recombination rates among and within chromosomes, which associate positively with nucleotide diversity and GC content and negatively with chromosome size. Recombination rates increased significantly at regulatory regions but not necessarily at gene bodies. CpG islands are associated strongly with recombination rates, though their specific position and local DNA methylation patterns likely influence this relationship. The association with retrotransposons varied according to specific family and location. Our results also provide evidence of heterogeneous intrachromosomal conservation of recombination maps between the blackcap and its closest sister taxon, the garden warbler. These findings highlight the considerable variability of recombination rates at different scales and the role of specific genomic features in shaping this variation. This study opens the possibility of further investigating the impact of recombination on specific population-genomic features.

Genome organization of major tandem repeats and their specificity for heterochromatin of macro- and microchromosomes in Japanese quail

Tandemly repeated DNAs form heterochromatic regions of chromosomes, including the vital centromeric chromatin. Despite the progress in new genomic technologies tandem repeats remain poorly deciphered and need targeted analysis in the species of interest. The Japanese quail is one of the highest-producing poultry species as well as a model organism. Its genome differs by a noticeable accumulation of heterochromatin, which led to an increase by 1/7 compared to the chicken genome size. Prominent heterochromatin blocks occupy the short arms of acrocentric macrochromosomes and of microchromosomes. We have applied de novo repeat finder approach to unassembled raw reads of the Japanese quail genome. We identified the 20 most common tandem repeats with the abundance >1 Mb, which represent about 4.8% of the genome. We found that tandem repeat CjapSAT primarily contribute to the centromeric regions of the macrochromosomes CJA1-8. Cjap31B together with previously characterized BglII make up centromere regions of microchromosomes and W chromosome. Other repeats populate heterochromatin of microchromosomal short arms in unequal proportions, as revealed by FISH. The Cjap84A, Cjap408A and CjapSAT repeat sequences show similarities with retrotransposon motifs. This suggests that retroelements may have played a crucial role in the distribution of repeats throughout the Japanese quail genome.

Mendelian nightmares: the germline-restricted chromosome of songbirds

Germline-restricted chromosomes (GRCs) are accessory chromosomes that occur only in germ cells. They are eliminated from somatic cells through programmed DNA elimination during embryo development. GRCs have been observed in several unrelated animal taxa and show peculiar modes of non-Mendelian inheritance and within-individual elimination. Recent cytogenetic and phylogenomic evidence suggests that a GRC is present across the species-rich songbirds, but absent in non-passerine birds, implying that over half of all 10,500 bird species have extensive germline/soma genome differences. Here, we review recent insights gained from genomic, transcriptomic, and cytogenetic approaches with regard to the genetic content, phylogenetic distribution, and inheritance of the songbird GRC. While many questions remain unsolved in terms of GRC inheritance, elimination, and function, we discuss plausible scenarios and future directions for understanding this widespread form of programmed DNA elimination.

Meiotic chromosome organization and crossover patterns

Meiosis is the foundation of sexual reproduction, and crossover recombination is one hallmark of meiosis. Crossovers establish the physical connections between homolog chromosomes (homologs) for their proper segregation and exchange DNA between homologs to promote genetic diversity in gametes and thus progenies. Aberrant crossover patterns, e.g. absence of the obligatory crossover, are the leading cause of infertility, miscarriage, and congenital disease. Therefore, crossover patterns have to be tightly controlled. During meiosis, loop/axis organized chromosomes provide the structural basis and regulatory machinery for crossover patterning. Accumulating evidence shows that chromosome axis length regulates not only the numbers but also the positions of crossovers. In addition, recent studies suggest that alterations in axis length and the resultant alterations in crossover frequency may contribute to evolutionary adaptation. Here, current advances regarding these issues are reviewed, the possible mechanisms for axis length regulating crossover frequency are discussed, and important issues that need further investigations are suggested.

Why Do Some Vertebrates Have Microchromosomes?

With more than 70,000 living species, vertebrates have a huge impact on the field of biology and research, including karyotype evolution. One prominent aspect of many vertebrate karyotypes is the enigmatic occurrence of tiny and often cytogenetically indistinguishable microchromosomes, which possess distinctive features compared to macrochromosomes. Why certain vertebrate species carry these microchromosomes in some lineages while others do not, and how they evolve remain open questions. New studies have shown that microchromosomes exhibit certain unique characteristics of genome structure and organization, such as high gene densities, low heterochromatin levels, and high rates of recombination. Our review focuses on recent concepts to expand current knowledge on the dynamic nature of karyotype evolution in vertebrates, raising important questions regarding the evolutionary origins and ramifications of microchromosomes. We introduce the basic karyotypic features to clarify the size, shape, and morphology of macro- and microchromosomes and report their distribution across different lineages. Finally, we characterize the mechanisms of different evolutionary forces underlying the origin and evolution of microchromosomes.

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.

A test for meiotic drive in hybrids between Australian and Timor zebra finches

Meiotic drivers have been proposed as a potent evolutionary force underlying genetic and phenotypic variation, genome structure, and also speciation. Due to their strong selective advantage, they are expected to rapidly spread through a population despite potentially detrimental effects on organismal fitness. Once fixed, autosomal drivers are cryptic within populations and only become visible in between-population crosses lacking the driver or corresponding suppressor. However, the assumed ubiquity of meiotic drivers has rarely been assessed in crosses between populations or species. Here we test for meiotic drive in hybrid embryos and offspring of Timor and Australian zebra finches-subspecies that have evolved in isolation for about two million years-using 38,541 informative transmissions of 56 markers linked to either centromeres or distal chromosome ends. We did not find evidence for meiotic driver loci on specific chromosomes. However, we observed a weak overall transmission bias toward Timor alleles at centromeres in females (transmission probability of Australian alleles of 47%, nominal p = 6 × 10-5). While this is in line with the centromere drive theory, it goes against the expectation that the subspecies with the larger effective population size (i.e., the Australian zebra finch) should have evolved the more potent meiotic drivers. We thus caution against interpreting our finding as definite evidence for centromeric drive. Yet, weak centromeric meiotic drivers may be more common than generally anticipated and we encourage further studies that are designed to detect also small effect meiotic drivers.

MLH1 focus mapping in the guinea fowl (Numida meleagris) give insights into the crossover landscapes in birds

Crossover rates and localization are not homogeneous throughout the genomes. Along the chromosomes of almost all species, domains with high crossover rates alternate with domains where crossover rates are significantly lower than the genome-wide average. The distribution of crossovers along chromosomes constitutes the recombination landscape of a given species and can be analyzed at broadscale using immunostaining of the MLH1 protein, a component of mature recombination nodules found on synaptonemal complexes during pachytene. We scored the MLH1 foci in oocytes of the chicken and the guinea fowl and compared their frequencies in the largest bivalents. The average autosomal number of foci is 62 in the chicken and 44 in the guinea fowl. The lower number in the guinea fowl responds to the occurrence of fewer crossovers in the six largest bivalents, where most MLH1 foci occur within one-fifth of the chromosome length with high polarization towards opposite ends. The skewed distribution of foci in the guinea fowl contrast with the more uniform distribution of numerous foci in the chicken, especially in the four largest bivalents. The crossover distribution observed in the guinea fowl is unusual among Galloanserae and also differs from other, more distantly related birds. We discussed the current evidence showing that the shift towards crossover localization, as observed in the guinea fowl, was not a unique event but also occurred at different moments of bird evolution. A comparative analysis of genome-wide average recombination rates in birds shows variations within narrower limits compared to mammals and the absence of a phylogenetic trend.

Heterochiasmy and Sexual Dimorphism: The Case of the Barn Swallow (Hirundo rustica, Hirundinidae, Aves)

Heterochiasmy, a sex-based difference in recombination rate, has been detected in many species of animals and plants. Several hypotheses about evolutionary causes of heterochiasmy were proposed. However, there is a shortage of empirical data. In this paper, we compared recombination related traits in females and males of the barn swallow Hirundo rustica (Linnaeus, 1758), the species under strong sexual selection, with those in the pale martin Riparia diluta (Sharpe and Wyatt, 1893), a related and ecologically similar species with the same karyotype (2N = 78), but without obvious sexual dimorphism. Recombination traits were examined in pachytene chromosome spreads prepared from spermatocytes and oocytes. Synaptonemal complexes and mature recombination nodules were visualized with antibodies to SYCP3 and MLH1 proteins, correspondingly. Recombination rate was significantly higher (p = 0.0001) in barn swallow females (55.6 ± 6.3 recombination nodules per autosomal genome), caused by the higher number of nodules at the macrochromosomes, than in males (49.0 ± 4.5). They also showed more even distribution of recombination nodules along the macrochromosomes. At the same time, in the pale martin, sexual differences in recombination rate and distributions were rather small. We speculate that an elevated recombination rate in the female barn swallows might have evolved as a compensatory reaction to runaway sexual selection in males.

Sex Differences in the Recombination Landscape

Sex differences in overall recombination rates are well known, but little theoretical or empirical attention has been given to how and why sexes differ in their recombination landscapes: the patterns of recombination along chromosomes. In the first scientific review of this phenomenon, we find that recombination is biased toward telomeres in males and more uniformly distributed in females in most vertebrates and many other eukaryotes. Notable exceptions to this pattern exist, however. Fine-scale recombination patterns also frequently differ between males and females. The molecular mechanisms responsible for sex differences remain unclear, but chromatin landscapes play a role. Why these sex differences evolve also is unclear. Hypotheses suggest that they may result from sexually antagonistic selection acting on coding genes and their regulatory elements, meiotic drive in females, selection during the haploid phase of the life cycle, selection against aneuploidy, or mechanistic constraints. No single hypothesis, however, can adequately explain the evolution of sex differences in all cases. Sex-specific recombination landscapes have important consequences for population differentiation and sex chromosome evolution.

Interbreed variation in meiotic recombination rate and distribution in the domestic chicken Gallus gallus

The efficiency of natural and artificial selection is critically dependent on the recombination rate. However, interbreed and individual variation in recombination rate in poultry remains unknown. Conventional methods of analysis of recombination such as genetic linkage analysis, sperm genotyping and chiasma count at lampbrush chromosomes are expensive and time-consuming. In this study, we analyzed the number and distribution of recombination nodules in spermatocytes of the roosters of six chicken breeds using immunolocalization of key proteins involved in chromosome pairing and recombination. We revealed significant effects of breed (R 2 = 0.17 ; p < 0.001 ) and individual (R 2 = 0.28 ; p < 0.001 ) on variation in the number of recombination nodules. Both interbreed and individual variations in recombination rate were almost entirely determined by variation in recombination density on macrochromosomes, because almost all microchromosomes in each breed had one recombination nodule. Despite interbreed differences in the density of recombination nodules, the patterns of their distribution along homologous chromosomes were similar. The breeds examined in this study showed a correspondence between the age of the breed and its recombination rate. Those with high recombination rates (Pervomai, Russian White and Brahma) are relatively young breeds created by crossing several local breeds. The breeds displaying low recombination rate are ancient local breeds: Cochin (Indo-China), Brown Leghorn (Tuscany, Italy) and Russian Crested (the European part of Russia).

Germline-restricted chromosome (GRC) is widespread among songbirds

An unusual supernumerary chromosome has been reported for two related avian species, the zebra and Bengalese finches. This large, germline-restricted chromosome (GRC) is eliminated from somatic cells and spermatids and transmitted via oocytes only. Its origin, distribution among avian lineages, and function were mostly unknown so far. Using immunolocalization of key meiotic proteins, we found that GRCs of varying size and genetic content are present in all 16 songbird species investigated and absent from germline genomes of all eight examined bird species from other avian orders. Results of fluorescent in situ hybridization of microdissected GRC probes and their sequencing indicate that GRCs show little homology between songbird species and contain a variety of repetitive elements and unique sequences with paralogs in the somatic genome. Our data suggest that the GRC evolved in the common ancestor of all songbirds and underwent significant changes in the extant descendant lineages.

Amphibian and Avian Karyotype Evolution: Insights from Lampbrush Chromosome Studies

Amphibian and bird karyotypes typically have a complex organization, which makes them difficult for standard cytogenetic analysis. That is, amphibian chromosomes are generally large, enriched with repetitive elements, and characterized by the absence of informative banding patterns. The majority of avian karyotypes comprise a small number of relatively large macrochromosomes and numerous tiny morphologically undistinguishable microchromosomes. A good progress in investigation of amphibian and avian chromosome evolution became possible with the usage of giant lampbrush chromosomes typical for growing oocytes. Due to the giant size, peculiarities of organization and enrichment with cytological markers, lampbrush chromosomes can serve as an opportune model for comprehensive high-resolution cytogenetic and cytological investigations. Here, we review the main findings on chromosome evolution in amphibians and birds that were obtained using lampbrush chromosomes. In particular, we discuss the data on evolutionary chromosomal rearrangements, accumulation of polymorphisms, evolution of sex chromosomes as well as chromosomal changes during clonal reproduction of interspecies hybrids.

Broad-scale recombination pattern in the primitive bird Rhea americana (Ratites, Palaeognathae)

Birds have genomic and chromosomal features that make them an attractive group to analyze the evolution of recombination rate and the distribution of crossing over. Yet, analyses are biased towards certain species, especially domestic poultry and passerines. Here we analyze for the first time the recombination rate and crossover distribution in the primitive ratite bird, Rhea americana (Rheiformes, Palaeognathae). Using a cytogenetic approach for in situ mapping of crossovers we found that the total genetic map is 3050 cM with a global recombination rate of 2.1 cM/Mb for female rheas. In the five largest macrobivalents there were 3 or more crossovers in most bivalents. Recombination rates for macrobivalents ranges between 1.8-2.1 cM/Mb and the physical length of their synaptonemal complexes is highly predictive of their genetic lengths. The crossover rate at the pseudoautosomal region is 2.1 cM/Mb, similar to those of autosomal pairs 5 and 6 and only slightly higher compared to other macroautosomes. It is suggested that the presence of multiple crossovers on the largest macrobivalents is a feature common to many avian groups, irrespective of their position throughout phylogeny. These data provide new insights to analyze the heterogeneous recombination landscape of birds.

Immunocytological analysis of meiotic recombination in two anole lizards (Squamata, Dactyloidae)

Although the evolutionary importance of meiotic recombination is not disputed, the significance of interspecies differences in the recombination rates and recombination landscapes remains under-appreciated. Recombination rates and distribution of chiasmata have been examined cytologically in many mammalian species, whereas data on other vertebrates are scarce. Immunolocalization of the protein of the synaptonemal complex (SYCP3), centromere proteins and the mismatch-repair protein MLH1 was used, which is associated with the most common type of recombination nodules, to analyze the pattern of meiotic recombination in the male of two species of iguanian lizards, Anolis carolinensis Voigt, 1832 and Deiroptyx coelestinus (Cope, 1862). These species are separated by a relatively long evolutionary history although they retain the ancestral iguanian karyotype. In both species similar and extremely uneven distributions of MLH1 foci along the macrochromosome bivalents were detected: approximately 90% of crossovers were located at the distal 20% of the chromosome arm length. Almost total suppression of recombination in the intermediate and proximal regions of the chromosome arms contradicts the hypothesis that "homogenous recombination" is responsible for the low variation in GC content across the anole genome. It also leads to strong linkage disequilibrium between the genes located in these regions, which may benefit conservation of co-adaptive gene arrays responsible for the ecological adaptations of the anoles.

Microchromosome polymorphism in the sand lizard, Lacerta agilis Linnaeus, 1758 (Reptilia, Squamata)

Most true lizards (Lacertidae) share a conservative karyotype, consisting of 18 pairs of macrochromosomes and one microchromosome pair. Homeologues of the microchromosome are present in other squamates and even in chickens. No structural autosomal microchromosome polymorphisms have been described previously in lizards. We found homozygous and heterozygous carriers of a microchromosome variant in a Siberian population of the sand lizard, Lacerta agilis Linnaeus, 1758. The variant microchromosome was almost twice as long as the standard one. In heterozygotes at pachytene, the microchromosomes firstly pair in proximal regions and the central part of the longer axial element undergoes foldback synapsis, then its distal region pairs with the distal region of the standard partner. At metaphase-I, the heteromorphic microchromosome bivalents have a proximal chiasma. The content of the additional segment was Ag-NOR, C-like DAPI, CMA3 negative. FISH with telomere PNA probe did not detect interstitial (TTAGGG)_n sequences in the heteromorphic and any other bivalents. Both homo- and heterozygous carriers were phenotypically normal. The presence of homozygotes shows that heterozygotes are fertile. Reduction in the number of microchromosomes is a clear trend in squamate evolution, as a result of microchromosomes fusing together or with macrochromosomes. Our findings indicate that gaining additional DNA may lead to a transformation of microchromosomes into small macrochromosomes without fusion.

Mapping centromeres of microchromosomes in the zebra finch (Taeniopygia guttata) using half-tetrad analysis

Centromeres usually consist of hundreds of kilobases of repetitive sequence which renders them difficult to assemble. As a consequence, centromeres are often missing from assembled genomes and their locations on physical chromosome maps have to be inferred from flanking sequences via fluorescence in situ hybridization (FISH). Alternatively, centromere positions can be mapped using linkage analyses in accidentally triploid individuals formed by half-tetrads (resulting from the inheritance of two chromatids from a single meiosis). The current genome assembly of the zebra finch (Taeniopygia guttata) comprises 32 chromosomes, but only for the ten largest chromosomes centromere positions have been mapped using FISH. We here map the positions of most of the remaining centromeres using half-tetrad analyses. For this purpose, we genotyped 37 zebra finches that were triploid or tetraploid due to inheritance errors (and mostly died as embryos) together with their parents at 64 microsatellite markers (at least two per chromosome). Using the information on centromere positions on the ten largest chromosomes, we were able to identify 12 cases of non-disjunction in maternal meiosis I and 10 cases of non-disjunction in maternal meiosis II. These 22 informative cases allowed us to infer centromere positions on additional 19 microchromosomes in reference to the current genome assembly. This knowledge will be valuable for studies of chromosome evolution, meiotic drive and species divergence in the avian lineage.

Meiotic recombination analysis in female ducks (Anas platyrhynchos)

Meiotic recombination in female ducks was directly studied by immunolocalization of MLH1 protein, a mismatch repair protein of mature recombination nodules. In total, 6820 crossovers were scored along the autosomal synaptonemal complexes in 122 meiotic nuclei. From this analysis we predict that the female map length of the duck is 2845 cM, with a genome wide recombination rate of 2 cM/Mb. MLH1-focus mapping along the six largest bivalents shows regional variations of recombination frequencies that can be linked to differences in chromosome morphology. From this MLH1 mapping it can be inferred that distally located markers will appear more separated in genetic maps than physically equidistant markers located near the centromeres on bivalents 1 and 2. Instead, markers at interstitial positions on the acrocentric bivalents 3-6 will appear more tightly linked than expected on the basis of their physical distance because recombination is comparatively lower at the mid region of these chromosomes. The present results provide useful information to complement linkage mapping in ducks and extend previous knowledge about the variation of recombination rates among domestic Galloanserae.

Heterologous Synapsis and Crossover Suppression in Heterozygotes for a Pericentric Inversion in the Zebra Finch

In the zebra finch, 2 alternative morphs regarding centromere position were described for chromosome 6. This polymorphism was interpreted to be the result of a pericentric inversion, but other causes of the centromere repositioning were not ruled out. We used immunofluorescence localization to examine the distribution of MLH1 foci on synaptonemal complexes to test the prediction that pericentric inversions cause synaptic irregularities and/or crossover suppression in heterozygotes. We found complete suppression of crossing over in the region involved in the rearrangement in male and female heterozygotes. In contrast, the same region showed high levels of crossing over in homozygotes for the acrocentric form of this chromosome. No inversion loops or synaptic irregularities were detected along bivalent 6 in heterozygotes suggesting that heterologous pairing is achieved during zygotene or early pachytene. Altogether these findings strongly indicate that the polymorphic chromosome 6 originated by a pericentric inversion. Since inversions are common rearrangements in karyotypic evolution in birds, it seems likely that early heterologous pairing could help to fix these rearrangements, preventing crossing overs in heterozygotes and their deleterious effects on fertility.

Sex-specific recombination maps for individual macrochromosomes in the Japanese quail (Coturnix japonica)

Meiotic recombination in the Japanese quail was directly studied by immunolocalization of mutL homolog 1 (MLH1), a mismatch repair protein of mature recombination nodules. In total, 15,862 crossovers were scored along the autosomal synaptonemal complexes in 308 meiotic nuclei from males and females. Crossover frequencies calculated from MLH1 foci show wide similitude between males and females with slightly higher number of foci in females. From this analysis, we predict that the sex-averaged map length of the Japanese quail is 2580 cM, with a genome-wide recombination rate of 1.9 cM/Mb. MLH1 focus mapping along the six largest bivalents showed few intersex differences in the distribution of crossovers along with variant patterns in metacentric and acrocentric macrobivalents. These results provide valuable information to complement linkage map analysis in the species while providing insight into our understanding of the mechanisms of crossover distribution along chromosome arms.

Strong artificial selection in domestic mammals did not result in an increased recombination rate

Recombination rates vary in intensity and location at the species, individual, sex and chromosome levels. Despite the fundamental biological importance of this process, the selective forces that operate to shape recombination rate and patterns are unclear. Domestication offers a unique opportunity to study the interplay between recombination and selection. In domesticates, intense selection for particular traits is imposed on small populations over many generations, resulting in organisms that differ, sometimes dramatically, in morphology and physiology from their wild ancestor. Although earlier studies suggested increased recombination rate in domesticates, a formal comparison of recombination rates between domestic mammals and their wild congeners was missing. In order to determine broad-scale recombination rate, we used immunolabeling detection of MLH1 foci as crossover markers in spermatocytes in three pairs of closely related wild and domestic species (dog and wolf, goat and ibex, and sheep and mouflon). In the three pairs, and contrary to previous suggestions, our data show that contemporary recombination rate is higher in the wild species. Subsequently, we inferred recombination breakpoints in sequence data for 16 genomic regions in dogs and wolves, each containing a locus associated with a dog phenotype potentially under selection during domestication. No difference in the number and distribution of recombination breakpoints was found between dogs and wolves. We conclude that our data indicate that strong directional selection did not result in changes in recombination in domestic mammals, and that both upper and lower bounds for crossover rates may be tightly regulated.

Diverse stages of sex-chromosome differentiation in tinamid birds: evidence from crossover analysis in Eudromia elegans and Crypturellus tataupa

All extant birds share the same sex-chromosome system: ZZ males and ZW females with striking differences in the stages of sex-chromosome differentiation between the primitive palaeognathus ratites and the large majority of avian species grouped within neognaths. Evolutionarily close to ratites is the neotropical order Tinamiformes that has been scarcely explored regarding their ZW pair morphology and constitution. Tinamous, when compared to ratites, constitute a large group among Palaeognathae, therefore, exploring the extent of homology between the Z and W chromosomes in this group might reveal key features on the evolution of the avian sex chromosomes. We mapped MLH1 foci that are crossover markers on pachytene bivalents to determine the size and localization of the homologous region shared by the Z and W chromosomes in two tinamous: Eudromia elegans and Crypturellus tataupa. We found that the homologous (pseudoautosomal) region differ significantly in size between these two species. They both have a single recombination event on the long arm of the acrocentric Z and W chromosomes. However, in E. elegans the pseudoautosomal region occupies one-fourth of the W chromosome, while in C. tataupa it is restricted to the tip of the long arm of the W. The W chromosomes in these two species differ in their heterochromatin content: in E. elegans it shows a terminal euchromatic segment and in C. tataupa is completely heterochromatic. These results show that tinamous have ZW pairs with more diversified stages of differentiation compared to ratites. Finally, the idea that the avian proto-sex chromosomes started to diverge from the end of the long arm towards the centromere of an acrocentric pair is discussed.

A comparison of SNPs and microsatellites as linkage mapping markers: lessons from the zebra finch (Taeniopygia guttata)

BACKGROUND

Genetic linkage maps are essential tools when searching for quantitative trait loci (QTL). To maximize genome coverage and provide an evenly spaced marker distribution a combination of different types of genetic marker are sometimes used. In this study we created linkage maps of four zebra finch (Taeniopygia guttata) chromosomes (1, 1A, 2 and 9) using two types of marker, Single Nucleotide Polymorphisms (SNPs) and microsatellites. To assess the effectiveness and accuracy of each kind of marker we compared maps built with each marker type separately and with both types of marker combined. Linkage map marker order was validated by making comparisons to the assembled zebra finch genome sequence.

RESULTS

We showed that marker order was less reliable and linkage map lengths were inflated for microsatellite maps relative to SNP maps, apparently due to differing error rates between the two types of marker. Guidelines on how to minimise the effects of error are provided. In particular, we show that when combining both types of marker the conventional process of building linkage maps, whereby the most informative markers are added to the map first, has to be modified in order to improve map accuracy.

CONCLUSIONS

When using multiple types and large numbers of markers to create dense linkage maps, the least error prone loci (SNPs) rather than the most informative should be used to create framework maps before the addition of other potentially more error prone markers (microsatellites). This raises questions about the accuracy of marker order and predicted recombination rates in previous microsatellite linkage maps which were created using the conventional building process, however, provided suitable error detection strategies are followed microsatellite-based maps can continue to be regarded as reasonably reliable.

Pronounced inter- and intrachromosomal variation in linkage disequilibrium across the zebra finch genome

The extent of nonrandom association of alleles at two or more loci, termed linkage disequilibrium (LD), can reveal much about population demography, selection, and recombination rate, and is a key consideration when designing association mapping studies. Here, we describe a genome-wide analysis of LD in the zebra finch (Taeniopygia guttata) using 838 single nucleotide polymorphisms and present LD maps for all assembled chromosomes. We found that LD declined with physical distance approximately five times faster on the microchromosomes compared to macrochromosomes. The distribution of LD across individual macrochromosomes also varied in a distinct pattern. In the center of the macrochromosomes there were large blocks of markers, sometimes spanning tens of mega bases, in strong LD whereas on the ends of macrochromosomes LD declined more rapidly. Regions of high LD were not simply the result of suppressed recombination around the centromere and this pattern has not been observed previously in other taxa. We also found evidence that this pattern of LD has remained stable across many generations. The variability in LD between and within chromosomes has important implications for genome wide association studies in birds and for our understanding of the distribution of recombination events and the processes that govern them.

A genetic linkage map for the saltwater crocodile (Crocodylus porosus)

Background

Genome elucidation is now in high gear for many organisms, and whilst genetic maps have been developed for a broad array of species, surprisingly, no such maps exist for a crocodilian, or indeed any other non-avian member of the Class Reptilia. Genetic linkage maps are essential tools for the mapping and dissection of complex quantitative trait loci (QTL), and in order to permit systematic genome scans for the identification of genes affecting economically important traits in farmed crocodilians, a comprehensive genetic linage map will be necessary.

Results

A first-generation genetic linkage map for the saltwater crocodile (Crocodylus porosus) was constructed using 203 microsatellite markers amplified across a two-generation pedigree comprising ten full-sib families from a commercial population at Darwin Crocodile Farm, Northern Territory, Australia. Linkage analyses identified fourteen linkage groups comprising a total of 180 loci, with 23 loci remaining unlinked. Markers were ordered within linkage groups employing a heuristic approach using CRIMAP v3.0 software. The estimated female and male recombination map lengths were 1824.1 and 319.0 centimorgans (cM) respectively, revealing an uncommonly large disparity in recombination map lengths between sexes (ratio of 5.7:1).

Conclusion

We have generated the first genetic linkage map for a crocodilian, or indeed any other non-avian reptile. The uncommonly large disparity in recombination map lengths confirms previous preliminary evidence of major differences in sex-specific recombination rates in a species that exhibits temperature-dependent sex determination (TSD). However, at this point the reason for this disparity in saltwater crocodiles remains unclear.

This map will be a valuable resource for crocodilian researchers, facilitating the systematic genome scans necessary for identifying genes affecting complex traits of economic importance in the crocodile industry. In addition, since many of the markers placed on this genetic map have been evaluated in up to 18 other extant species of crocodilian, this map will be of intrinsic value to comparative mapping efforts aimed at understanding genome content and organization among crocodilians, as well as the molecular evolution of reptilian and other amniote genomes. As researchers continue to work towards elucidation of the crocodilian genome, this first generation map lays the groundwork for more detailed mapping investigations, as well as providing a valuable scaffold for future genome sequence assembly.

Relationship between physical and genetic distances along the zebra finch Z chromosome

Nine bacterial artificial chromosomes containing genes linked to the Z chromosome of the zebra finch (Taeniopygia guttata) were localized using FISH on synaptonemal complex spreads. Their positions were correlated with those previously reported on the mitotic Z chromosome, showing a linear relationship between positions along the mitotic chromosome and its synaptonemal complex. Distances in cM between the genes were calculated using a cytological map of the crossing-over based on the distribution of MLH1 foci along the ZZ synaptonemal complex (MLH1-cM map). It is shown that physical and genetic distances lack a linear relationship along most of the chromosome length, due to clustering of crossover events around the telomeres. This relationship departs strongly from that observed in the chicken Z chromosome and reflects the existence of different recombination rates and patterns among birds in spite of wide genomic conservation.

Chromosomal abnormalities, meiotic behavior and fertility in domestic animals

Since the advent of the surface microspreading technique for synaptonemal complex analysis, increasing interest in describing the synapsis patterns of chromosome abnormalities associated with fertility of domestic animals has been noticed during the past three decades. In spite of the number of scientific reports describing the occurrence of structural chromosome abnormalities, their meiotic behavior and gametic products, little is known in domestic animal species about the functional effects of such chromosome aberrations in the germ cell line of carriers. However, some interesting facts gained from recent and previous studies on the meiotic behavior of chromosome abnormalities of domestic animals permit us to discuss, in the frame of recent knowledge emerging from mouse and human investigations, the possible mechanism implicated in the well known association between meiotic disruption and chromosome pairing failure. New cytogenetic techniques, based on molecular and immunofluorescent analyses, are allowing a better description of meiotic processes, including gamete production. The present communication reviews the knowledge of the meiotic consequences of chromosome abnormalities in domestic animals.

Recombination map of the common shrew, Sorex araneus (Eulipotyphla, Mammalia)

The Eurasian common shrew (Sorex araneus L.) is characterized by spectacular chromosomal variation, both autosomal variation of the Robertsonian type and an XX/XY(1)Y(2) system of sex determination. It is an important mammalian model of chromosomal and genome evolution as it is one of the few species with a complete genome sequence. Here we generate a high-precision cytological recombination map for the species, the third such map produced in mammals, following those for humans and house mice. We prepared synaptonemal complex (SC) spreads of meiotic chromosomes from 638 spermatocytes of 22 males of nine different Robertsonian karyotypes, identifying each autosome arm by differential DAPI staining. Altogether we mapped 13,983 recombination sites along 7095 individual autosomes, using immunolocalization of MLH1, a mismatch repair protein marking recombination sites. We estimated the total recombination length of the shrew genome as 1145 cM. The majority of bivalents showed a high recombination frequency near the telomeres and a low frequency near the centromeres. The distances between MLH1 foci were consistent with crossover interference both within chromosome arms and across the centromere in metacentric bivalents. The pattern of recombination along a chromosome arm was a function of its length, interference, and centromere and telomere effects. The specific DNA sequence must also be important because chromosome arms of the same length differed substantially in their recombination pattern. These features of recombination show great similarity with humans and mice and suggest generality among mammals. However, contrary to a widespread perception, the metacentric bivalent tu usually lacked an MLH1 focus on one of its chromosome arms, arguing against a minimum requirement of one chiasma per chromosome arm for correct segregation. With regard to autosomal chromosomal variation, the chromosomes showing Robertsonian polymorphism display MLH1 foci that become increasingly distal when comparing acrocentric homozygotes, heterozygotes, and metacentric homozygotes. Within the sex trivalent XY(1)Y(2), the autosomal part of the complex behaves similarly to other autosomes.

Sex: differences in mutation, recombination, selection, gene flow, and genetic drift

In many instances, there are large sex differences in mutation rates, recombination rates, selection, rates of gene flow, and genetic drift. Mutation rates are often higher in males, a difference that has been estimated both directly and indirectly. The higher male mutation rate appears related to the larger number of cell divisions in male lineages but mutation rates also appear gene- and organism-specific. When there is recombination in only one sex, it is always the homogametic sex. When there is recombination in both sexes, females often have higher recombination but there are many exceptions. There are a number of hypotheses to explain the sex differences in recombination. Sex-specific differences in selection may result in stable polymorphisms or for sex chromosomes, faster evolutionary change. In addition, sex-dependent selection may result in antagonistic pleiotropy or sexually antagonistic genes. There are many examples of sex-specific differences in gene flow (dispersal) and a number of adaptive explanations for these differences. The overall effective population size (genetic drift) is dominated by the lower sex-specific effective population size. The mean of the mutation, recombination, and gene flow rates over the two sexes can be used in a population genetics context unless there are sex-specific differences in selection or genetic drift. Sex-specific differences in these evolutionary factors appear to be unrelated to each other. The evolutionary explanations for sex-specific differences for each factor are multifaceted and, in addition, explanations may include chance, nonadaptive differences, or mechanistic, nonevolutionary factors.