A WIDESPREAD B CHROMOSOME POLYMORPHISM MAINTAINED WITHOUT APPARENT DRIVE

Interpopulation spread of a parasitic B chromosome is unlikely through males in the grasshopper Eyprepocnemis plorans

The near-neutral model of B chromosome evolution predicts that population invasion is quite fast. To test this prediction, in 1994, we introduced males of the grasshopper Eyprepocnemis plorans from a B-carrying population into a B-lacking population and monitored the evolution of B-chromosome frequency up to 2013. We observed fluctuating very low B frequency across years but, remarkably, the B chromosome introduced (the B2 variant) was found up to 1996 only, whereas the B1 variant was present from 1996 onwards, presumably introduced by fishermen using E. plorans males as bait. Effective introgression of genetic material from the donor population was evidenced by the presence of a satellite DNA on autosome 9 (up to 1999) and the presence of one individual in 2006 showing an ISSR marker profile being highly similar to that found in the donor population. This indicated that the males introduced by us effectively mated with resident females, but donor genes rapidly decreased in frequency after this non-recurrent migration event. Taken together, our results indicated: (i) that the non-recurrent migration event had a slight, transient genetic effect on the recipient population, which was diluted in only a few generations; and (ii) that even with recurrent migration (forced by fishermen) the B chromosome failed to increase in frequency. Bearing in mind that B chromosomes in this species drive through females only, we hypothesize that B chromosomes most likely failed invasion in both migration events because the migrating sex shows no B-drive.

The Modern View of B Chromosomes Under the Impact of High Scale Omics Analyses

Supernumerary B chromosomes (Bs) are extra karyotype units in addition to A chromosomes, and are found in some fungi and thousands of animals and plant species. Bs are uniquely characterized due to their non-Mendelian inheritance, and represent one of the best examples of genomic conflict. Over the last decades, their genetic composition, function and evolution have remained an unresolved query, although a few successful attempts have been made to address these phenomena. A classical concept based on cytogenetics and genetics is that Bs are selfish and abundant with DNA repeats and transposons, and in most cases, they do not carry any function. However, recently, the modern quantum development of high scale multi-omics techniques has shifted B research towards a new-born field that we call "B-omics". We review the recent literature and add novel perspectives to the B research, discussing the role of new technologies to understand the mechanistic perspectives of the molecular evolution and function of Bs. The modern view states that B chromosomes are enriched with genes for many significant biological functions, including but not limited to the interesting set of genes related to cell cycle and chromosome structure. Furthermore, the presence of B chromosomes could favor genomic rearrangements and influence the nuclear environment affecting the function of other chromatin regions. We hypothesize that B chromosomes might play a key function in driving their transmission and maintenance inside the cell, as well as offer an extra genomic compartment for evolution.

Transmission and Drive Involving Parasitic B Chromosomes

B chromosomes (Bs) are enigmatic additional elements in the genomes of thousands of species of plants, animals, and fungi. How do these non-essential, harmful, and parasitic chromosomes maintain their presence in their hosts, making demands on all the essential functions of their host genomes? The answer seems to be that they have mechanisms of drive which enable them to enhance their transmission rates by various processes of non-mendelian inheritance. It is also becoming increasingly clear that the host genomes are developing their own mechanisms to resist the impact of the harmful effects of the Bs.

Long-term monitoring of B-chromosome invasion and neutralization in a population of Prospero autumnale (Asparagaceae)

B chromosomes have been reported in about 15% of eukaryotes, but long-term dynamics of B chromosomes in a single natural population has rarely been analyzed. Prospero autumnale plants collected in 1981 and 1983 at Cuesta de La Palma population had shown the presence of B chromosomes. We analyze here seven additional samples collected between 1987 and 2015, and show that B frequency increased significantly during the 1980s and showed minor fluctuations between 2005 and 2015. A mother-offspring analysis of B chromosome transmission, at population level, showed significant drive on the male side (k_B  = 0.65) and significant drag on the female side (k_B  = 0.33), with average B transmission rate being very close to the Mendelian rate (0.5). No significant effects of B chromosomes were observed on a number of vigor and fertility-related traits. Within a parasite/host framework, these results suggest that B chromosomes' drive on the male side is the main pathway for B chromosome invasion, whereas B chromosome drag on the female side might be the main manifestation of host genome resistance in this species. Prospero autumnale thus illuminates a novel evolutionary pathway for B chromosome neutralization by means of a decrease in B transmission through the nondriving sex.

POLYMORPHISM REGENERATION FOR A NEUTRALIZED SELFISH B CHROMOSOME

Long-run evolution of B chromosomes is mainly made up by an evolutionary arms race between these selfish genetic elements and the standard genome. The suppression of B drive is one of the clearest expressions of genome defense against B chromosomes. After drive neutralization, the B is condemned to extinction unless a new variant showing drive can emerge and replace it. This paper reports the first empirical evidence for the substitution of a neutralized B variant by a new selfish B variant. Such a polymorphism regeneration has recently taken place in a natural population of the grasshopper Eyprepocnemis plorans.

Post-meiotic B chromosome expulsion, during spermiogenesis, in two grasshopper species

Most supernumerary (B) chromosomes are parasitic elements carrying out an evolutionary arms race with the standard (A) chromosomes. A variety of weapons for attack and defense have evolved in both contending elements, the most conspicuous being B chromosome drive and A chromosome drive suppression. Here, we show for the first time that most microspermatids formed during spermiogenesis in two grasshopper species contain expulsed B chromosomes. By using DNA probes for B-specific satellite DNAs in Eumigus monticola and Eyprepocnemis plorans, and also 18S rDNA in the latter species, we were able to count the number of B chromosomes in standard spermatids submitted to fluorescence in situ hybridization, as well as visualizing B chromosomes inside most microspermatids. In E. plorans, the presence of B-carrying microspermatids in 1B males was associated with a significant decrease in the proportion of B-carrying standard spermatids. The fact that this decrease was apparent in elongating spermatids but not in round ones demonstrates that meiosis yields 1:1 proportions of 0B and 1B spermatids and hence that B elimination takes place post-meiotically, i.e., during spermiogenesis, implying a 5-25% decrease in B transmission rate. In E. monticola, the B chromosome is mitotically unstable and B number varies between cells within a same individual. A comparison of B frequency between round and elongating spermatids of a same individual revealed a significant 12.3% decrease. We conclude that B chromosome elimination during spermiogenesis is a defense weapon of the host genome to get rid of parasitic chromosomes.

Spread of a new parasitic B chromosome variant is facilitated by high gene flow

The B₂₄ chromosome variant emerged several decades ago in a Spanish population of the grasshopper Eyprepocnemis plorans and is currently reaching adjacent populations. Here we report, for the first time, how a parasitic B chromosome (a strictly vertically transmitted parasite) expands its geographical range aided by high gene flow in the host species. For six years we analyzed B frequency in several populations to the east and west of the original population and found extensive spatial variation, but only a slight temporal trend. The highest B₂₄ frequency was found in its original population (Torrox) and it decreased closer to both the eastern and the western populations. The analysis of Inter Simple Sequence Repeat (ISSR) markers showed the existence of a low but significant degree of population subdivision, as well as significant isolation by distance (IBD). Pairwise N_em estimates suggested the existence of high gene flow between the four populations located in the Torrox area, with higher values towards the east. No significant barriers to gene flow were found among these four populations, and we conclude that high gene flow is facilitating B₂₄ diffusion both eastward and westward, with minor role for B₂₄ drive due to the arrival of drive suppressor genes which are also frequent in the donor population.

Temporal frequency stability and absence of effects on mating behaviour for an autosomal supernumerary segment in two natural populations of the grasshopper Eyprepocnemis plorans

Interannual evolution of a polymorphism for a supernumerary segment in the smallest autosome of the grasshopper Eyprepocnemis plorans has been analysed in two natural populations. The polymorphism seemed to be stable in both populations, despite its undertransmission through heterozygous females carrying B chromosomes. Analyses of the effects of the extra segment on mating behaviour failed to show differential mating success in any sex or consistent effects on mating pattern. These results are discussed in relation to the maintenance of this polymorphism in natural populations.

A single, recent origin of the accessory B chromosome of the grasshopper Eyprepocnemis plorans

B chromosomes are dispensable chromosomes found in >2000 eukaryotic species, usually behaving as genomic parasites. Most B chromosomes seem to be made up of the same kind of DNA sequences present in the A chromosomes. This sequence similarity makes it difficult to obtain specific molecular probes that may permit B-presence diagnosis without cytogenetic analysis. We have developed a sequence-characterized amplified region (SCAR) marker for B chromosomes in the grasshopper Eyprepocnemis plorans, which specifically amplifies a 1510-bp DNA fragment exclusively in B-carrying individuals. Fluorescent in situ hybridization and fiber FISH analyses showed that this marker is a tandemly repeated DNA sequence closely intermingled with 45S rDNA. PCR reactions showed the presence of SCAR-like sequences in the A chromosomes, but in two separate fragments, supporting the intraspecific origin of B chromosomes in this species. SCAR marker DNA sequence showed to be identical in B chromosome variants from several localities from Spain and Morocco, and it was very similar to those found in B chromosome variants from Greece and Armenia. This strongly suggests that this sequence was already present in the ancestral B chromosome of this species. In addition, the scarce sequence variation observed among several B variants from very distant populations suggests either a functional constraint or, more likely, a recent and unique origin for B chromosomes in this species.

Causes of B chromosome variant substitution in the grasshopper Eyprepocnemis plorans

We have analysed B chromosome frequency for three consecutive years, B transmission rate at population and individual levels, clutch size, egg fertility and embryo-adult viability in a natural population of the grasshopper Eyprepocnemis plorans containing two different B chromosome variants, i.e. B(2) and B(24), the second being derived from the first and having replaced it in nearby populations. From 2002 to 2003 the relative frequency of both variants changed, although the differences did not reach significance. A mother-offspring analysis showed no significant effect of any of the two B variants on clutch size, egg fertility or embryo-adult viability, but B(24) was more efficiently transmitted than B(2) through males from the 2002 season, which explains the observed frequency change. Controlled crosses, at individual level, showed significant drive through some females for B(24) but not for B(2), suggesting that this difference in transmission rate might also be important for the substitution process. The analysis of relative fitness for B(2) and B(24) carriers for all fitness components, as a whole, showed a significantly better performance of B(24)-carrying individuals, suggesting that the cumulative effect of these slight differences might contribute to the replacement of B(2) by B(24).

Host recombination is dependent on the degree of parasitism

Parasites and hosts are involved in a continuous coevolutionary process leading to genetic changes in both counterparts. To understand this process, it is necessary to track host responses, one of which could be an increase in sex and recombination, such as is proposed by the Red Queen hypothesis. In this theoretical framework, the inducible recombination hypothesis states that B-chromosomes (genome parasites that prosper in natural populations of many living beings) elicit an increase in host chiasma frequency that is favoured by natural selection because it increases the proportion of recombinant progeny, some of which could be resistant to both B-chromosome effects and B-accumulation in the germline. We have found a clear parallelism between host recombination and the evolutionary status of the B-chromosome polymorphism, which provides explicit evidence for inducible recombination and strong support for the Red Queen hypothesis.

Steep variation in mitochondrial DNA and B chromosomes among natural populations of Eyprepocnemis plorans (Acrididae)

Restriction enzyme analysis of the mitochondrial DNA of Eyprepocnemis plorans with 9 restriction enzymes revealed low variability--only EcoRI revealed any variability with 3 distinguishable digestion patterns, here named types I, II and III. The samples studied were collected from regions where different types of B chromosome exist. The demes from the central area show the B1 type (as a relict of a probably once continuous distribution) and also mtDNA type II. These show a parallel substitution towards the SW neighbouring demes by other Bs and mtDNA type I. However, the latter mtDNA type had a wider distribution and is predominant, or even the only one found in most other samples where other Bs exist. Considering the mtDNA as a marker unlinked with the Bs, some genetic differences should exist among areas defined by the B chromosomes. Our results support the hypothesis that in the central region the B chromosomes and the mtDNA are involved in two different events of substitution, but these do not necessarily occur in the same way.

The B-chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: II. Parasitic and neutralized B1 chromosomes

The transmission of the B1 chromosome through females has been analysed in three Moroccan populations (Smir, SO.DE.A. and Mechra) of the grasshopper Eyprepocnemis plorans. We analysed transmission ratio (kB) variation at two levels: intra-individual (to test female age effects) and inter-individual (to test for A chromosome effects). In 81.8% of females, kB did not differ among successive egg-pods, suggesting no effect of female age. The remaining females (18.2%), showed significant differences in kB values among egg-pods, but without clear temporal patterns. In Smir, kB ranged between B elimination (0.244) and B accumulation (0.689) but there was no net accumulation (mean +/- s.e. = 0.463 +/- 0.045). In SO.DE.A., all females analysed transmitted B1 at a Mendelian rate, with a mean kBequal to 0.512 +/- 0.020. In Mechra, kB ranged from 0.341 to 0.972, with mean kB (0.575 +/- 0.029) showing a net B accumulation. All these observations suggest that the B1 chromosome could be at a drive-suppression stage in Smir and Mechra, but that it has already been neutralised in SO.DE.A.

Fitness effect analysis of a heterochromatic supernumerary segment in the grasshopper Eyprepocnemis plorans

Several components of fitness were analysed in relation to the presence of a supernumerary chromosome segment (SCS) in two natural populations of the grasshopper Eyprepocnemis plorans, including clutch size, egg fertility, egg and embryo productivity and survivability from embryo to adult, and SCS transmission through males. The results have shown the absence of a significant relationship between SCS presence and these fitness components, with the single exception of egg fertility which decreases significantly in SCS females with mating shortage. This fertility decrease is thus expected to be relevant for the population dynamics of the SCS only in low-density populations, those in which it is difficult for females to find a male to copulate with before each egg-batch is ready to be laid. The analysis of the SCS transmission through males showed no significant differences between expected and observed SCS frequencies. The SCS polymorphism seems to be at a status close to neutrality in respect to fitness, but its slight disadvantage in transmission through females carrying B chromosomes predicts that the polymorphism should tend to disappear, unless SCS recurrent amplification, or another undiscovered force, counteracts this tendency.

Robertsonian translocations and B chromosomes in the Wellington tree weta, Hemideina crassidens (Orthoptera: Anostostomatidae)

Two karyotypes within the species Hemideina crassidens are described, 2n = 15 (XO) and 2n = 19 (XO). These two karyotypes have a NF of 28. The 19-karyotype was found exclusively in the southern part of the species range and the 15-karyotype was found in the north. The differences between the two karyotypes are interpreted as arising from two Robertsonian translocations (fission/fusion). Laboratory matings between weta with the two karyotypes produced viable offspring. During meiosis in F1 intraspecific hybrids metacentric and acrocentric autosomes aligned to form two trivalents, confirming homologies predicted by Robertsonian translocations. The subspecies H. c. crassicruris, (confined to Stephens Island) was found to be polymorphic for a metacentric B chromosome. An unusual association of sex and presence of B chromosome was observed in this island population with Bs found only in male weta.

B-chromosome evolution

B chromosomes are extra chromosomes to the standard complement that occur in many organisms. They can originate in a number of ways including derivation from autosomes and sex chromosomes in intra- and interspecies crosses. Their subsequent molecular evolution resembles that of univalent sex chromosomes, which involves gene silencing, heterochromatinization and the accumulation of repetitive DNA and transposons. B-chromosome frequencies in populations result from a balance between their transmission rates and their effects on host fitness. Their long-term evolution is considered to be the outcome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the generation of new variants. Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.

Common origin of B chromosome variants in the grasshopper eyprepocnemis plorans

Nine B chromosome variants, from seven different populations of Eyprepocnemis plorans collected at four localities in Spain and three in Morocco, have been shown to be mainly composed of two DNA sequences, i.e. a 180-bp tandem repeat and ribosomal DNA. B types, however, differ in the relative amounts of the two sequences. The most widespread one (B1) bears about the same amount of rDNA and 180 bp repeat, but three other variants that have reached a polymorphism by replacing B1 in smaller areas (B2, B5 and B24) carry a conspicuously larger amount of the 180 bp repeat. In Morocco, the most widespread B variant is also B1, and a rare variant that appeared in a single individual is also built with the same two DNA sequences. All these data point to a common origin for these B chromosomes, with B1 probably being the original one. The origin of the different B types and the possible relationship of the relative amount of 180 bp DNA repeat with B drive are discussed.

The B chromosome polymorphism of the grasshopper eyprepocnemis plorans in north africa. I. B variants and frequency

Polymorphism for B chromosomes has been detected in all nine populations of the grasshopper Eyprepocnemis plorans ssp. plorans sampled in Morocco. The most frequent B chromosome in all populations showed a C-banding pattern and size similar to those of the B1 variant found in the Iberian Peninsula. In addition, other B chromosome variants (B1iso1, B1iso2, B1d1, B1di1, B3 and B1dd1) were discovered in these populations, although at a very low frequency. No significant differences in B chromosome frequency were found either in the nine populations or, for some of them, in up to three consecutive years. These results are discussed in the light of current hypotheses on the evolution of this B chromosome polymorphism in the Iberian Peninsula.

Chromosomal strategies for adaptation to univalency

The orientation and segregation behaviour of different types of univalents, namely sex chromosomes, B chromosomes and autosomal univalents, were analysed in living spermatocytes of eight evolutionarily distant grasshopper species. The meiotic behaviour of each univalent was characterized in terms of velocity of prometaphase movements, frequency of reorientations, types of final orientation at metaphase I and modes of segregation at anaphase I. All these features were found to vary between different univalents. Certain combinations of these traits, defining a 'chromosomal strategy', appear commonly together in certain chromosome types, indicating that they are the result of selection acting on the chromosomes to increase transmission effectiveness. The sex univalents show in general a strategy in which all the features favouring an eventual equational segregation at anaphase I tend to be minimized. There is much more variation in behaviour among B chromosomes than among X chromosomes, which is a reflection of their heterogeneous nature. Induced autosomal univalents are studied in Locusta migratoria. They show a very irregular behaviour, indicating their lack of adaptation to univalency.

Sex and conflict

Evolutionary conflict occurs when the deterministic spread of an allele lowers the fitness either of its bearer or of other individuals in the population, leading to selection for suppressors. Sex promotes conflict because associations between alleles are temporary. Differing selection on males and females, sexual selection, and differences in transmission patterns between classes of nuclear and cytoplasmic genes can all give rise to conflict. Inert Y chromosomes, uniparental inheritance of cytoplasmic genes, mating strains and sexes, and many features of sexual behavior may have evolved in part as a result of evolutionary conflict. Estimates of its quantitative importance, however, are still needed.

Geographical distribution of B chromosomes in the grasshopper Eyprepocnemis plorans, along a river basin, is mainly shaped by non-selective historical events

The analysis of 19 populations of the grasshopper Eyprepocnemis plorans, collected along four rivers belonging to the Segura basin (Mundo, Benamor, Taibilla and Segura itself), has shown that the presence of B chromosomes ends abruptly in each river, coinciding with the existence of a narrow pass in which this grasshopper cannot live because of the absence of the appropriate habitats. The existence of a broad inland region lacking grasshoppers with B chromosomes suggests that B chromosomes arose after the first colonization of the Iberian Peninsula by E. plorans specimens from North African populations. The B chromosome seems to have spread upstream along each of these four rivers until reaching geographical barriers that have impeded its advance and thus have preserved the non-B chromosome region. The available evidence indicates that the observed geographical distribution of the B chromosome polymorphism in this zone was shaped mainly by historical non-selective events.

The inheritance of B chromosomes in Allium schoenoprasum L

The inheritance of B chromosomes has been investigated. In Allium schoenoprasum from the River Wye, Powys; controlled crosses between plants of known B chromosome constitution were carried out, and the numbers of Bs present in the progenies scored. There was considerable heterogeneity in B chromosome transmission rates across the range of B chromosome numbers, through both the male and female lines, and in crosses involving odd and even numbers of Bs. The mean B number of the progeny was lower than the Mendelian expectation in 41 of the 57 crosses, significantly so in about a third of cases. No progeny had a mean B number significantly higher than expected. The mean transmission rate per B was not significantly different between parents with different numbers of Bs, in either the male or female line, or between plants carrying odd or even numbers of Bs. Transmission through the male line (0.43) was higher than through the female line (0.35), but not significantly so. The overall mean transmission rate per B (0.4), estimated from all crosses, was significantly lower than the expected rate of 0.5. This study provides conclusive evidence that B chromosomes in A. schoenoprasum from the River Wye lack an effective accumulation mechanism and, furthermore, show significant losses during transmission to the progeny of controlled crosses.

Accidental twins in a monembryonic insect

A grasshopper egg was found containing two embryos that were both males and contained one B chromosome. Their development was delayed in comparison with that of the untwinned embryos from the same egg pod but their morphology was apparently normal. The possible mechanisms causing this unusual phenomenon are discussed. Key words : twins, twinning in insects, grasshoppers, Eyprepocnemis plorans.

Cloning and sequence analysis of an extremely homogeneous tandemly repeated DNA in the grasshopper Eyprepocnemis plorans

Digestion of total nuclear DNA of the grasshopper Eyprepocnemis plorans with seven different restriction endonucleases (REs), and subsequent agarose gel electrophoresis, has shown the presence of highly repetitive DNA yielding the typical ladder-like banding pattern. The most clear pattern was produced by DraI, the monomer being some 180 bp. This repeat unit was subsequently cloned and sequenced. Bidirectional sequencing of five randomly chosen clones showed exactly the same nucleotides in all 180 positions. The possible explanations for such an extreme homogeneity of this tandem repeat are discussed in the light of current hypotheses on repetitive DNA function and molecular drive mechanisms.

A comparative study of orientation at behavior of univalent in living grasshopper spermatocytes

Orientational movements and modes of segregation at anaphase I were analyzed in three different types of univalents in living spermatocytes of the grasshopper species Eyprepocnemis plorans, namely the sex univalent, three types of accessory chromosomes and spontaneous and induced autosomal univalents. When two or more univalents were present in the same spindle, their dynamics were directly compared. Chromosomes may show variable velocity and number of reorientations: the X and the most common B types (B1 and B2) are slow and rarely reorient, a more geographically restricted B (B5) is faster and reorients more often, and autosomal univalents are the fastest and show the highest frequency of reorientations. Nonetheless, the X and the accessories are rigorously reductional at anaphase I whereas autosomal univalents often fail to migrate or divide equationally. This indicates that orientational and segregational behavior are controlled mainly by chromosomal rather than cellular characteristics and that chromosomes may display a great variety of strategies to achieve regular segregation.

Possible origin of a B chromosome deduced from its DNA composition using double FISH technique

Double fluorescent in situ hybridization (FISH) with two DNA probes (a 180 bp tandemly repeated DNA and ribosomal DNA) was performed in embryo cells of the grasshopper Eyprepocnemis plorans. Repetitive DNA was present in most standard chromosomes (excepting 7, 8 and 10) and in the proximal two-thirds of the B chromosome, which was its major location in the complement. Ribosomal DNA was present distally on the B, and in the active nucleolar organizer regions (NORs) of the X, 9, 10 and 11 chromosomes. A small number of rRNA gene clusters was also observed in the pericentromeric regions of chromosomes 1-8. The double FISH technique showed that the B chromosome (B2 type) is mainly composed of a 180 bp tandem repeat and ribosomal DNA, the minute short arm being the only region that does not hybridize with them. The location and order of the centromere and both the DNA sequences on the B chromosome coincide only with those in the X chromosome, indicating that the B most probably derives from the X.