Harmful B-chromosomes in a mealy bug: additional evidence

The B-chromosomes (B's; supernumeraries) of the mealy bug, Pseudococcus obscurus Essig, segregate preferentially into the two functional products of male meiosis. This segregation thus serves as an accumulation mechanism. A cytological study of a population from Oakland, California, confirmed the results obtained earlier that the B's are harmful and are maintained only because of their accumulation mechanism. The wild females were studied directly. The number of B's in the males was determined by analyzing ten or more daughters of females without B's (0B females) after these were inseminated by wild males. The 0B females were exposed to the wild males in screen cages. The analysis of 4732 daughters of 231 caged females indicated that among the males which inseminated these females, there were 19.9% 0B males and the mean number of B's was 1.46 ± 0.07. Among 224 wild females which were collected at about the same time there were 12.5% 0B females, and the mean number of B's was 1.88 ± 0.09. Since the frequencies of the B's in the population changed only slightly from generation to generation, the expected zygotes of this generation were assumed to be similar to those from which both the males and the females developed. The expected zygotes were calculated from the observed frequencies of the B's among the sperm and the known rates of transmission in females. The zygotes were very similar to the females but quite different from the males. It was concluded, therefore, that the B's had little or no effect on the females carrying them, but reduced the fitness of the males. The fitness of the 0B, 1B, 2B, 3B and 4B males was calculated to be 1.00, 0.64, 0.56, 0.38 and 0.20 respectively. The rate of transmission of the B's decreased with the increase in the number of B's, from 0.84 in 1B males to 0.51 in 4B males. This decrease, and the decrease in male fitness with the increase in the number of B's are expected to help stabilize the number of B's in the population.This paper is dedicated to Professor Sally Hughes-Schrader on the occasion of her seventy-fifth birthday.Supported by grants GB 1585 and GB 6745 from the National Science Foundation, Washington, D. C.     

B-chromosomes in Iseilema laxum Hack. II. Genetics

Transmission of B-chromosomes to S₁ progenies and progenies of crosses involving parents with different numbers of B-chromosomes in Iseilema laxum Hack. is reported. The mean number of B's remained the same in S₁ progenies. In the progenies of the crosses, the mean number of B's increased and the range of their number in the offspring also is wider when they are transmitted through the male parent. The inheritance of B's is more or less normal on the female side. The phenomena of nondisjunction and precocious division of B's are more frequent in the male parent. Apart from nondisjunction and precocious divisions in the PMC's, there seems to be some post-meiotic mechanism responsible for the wide range and increase in the mean number of B's in crosses where the male parents contain B-chromosomes.     

B-chromosomes in Iseilema laxum Hack. I. Cytology

Upto 6 B-chromosomes occurred in natural populations of the fodder grass Iseilema laxum Hack. They were smaller than the A-chromosomes and stained darker. Although the number of B's varied in different PMC's of even the same anther presumably due to the phenomena of nondisjunction and precocious divisions, it was possible to assign a definite number to a particular plant. As seen from pachytene morphology, three types of B's were seen. More than one type can also occur in the same plant. Pairing can take place between homo-as well as heteromorphic B's resulting in bivalents. B-chromosomes in Iseilema laxum appear to have a common origin.     

B-Chromosomes inAllium flavum (Liliaceae) and some related species

B-Chromosomes ofAllium flavum, A. stamineum andA. carinatum were C-banded. InA. flavum different types of B's were found, one of them possessing a nucleolus organizer. In the B's ofA. flavum andA. stamineum the banding patterns resemble those found in the standard chromosomes. The B inA. carinatum is only terminally banded. InA. flavum chiasma localization in the B's appears to be dependent on C-band location, just as is the case in the A-chromosomes. An increase in B's was found to cause a small (but significant) increase in chiasma frequency in the PMC's. This may result from an alteration in the nucleotype by the B's.     

Mitotic instability leading to an accumulation of B-chromosomes in grasshoppers

The study of mitotically unstable B-chromosomes (supernumeraries) of two grasshopper species confirmed a suggestion made earlier (Nur, 1963) that the instability should always be associated with a tendency of the B's to increase in frequency. Among 780 Camnula pellucida (Scudder) males from California, 105 had B's. In the testes of these males the number of B's varied from follicle to follicle and ranged between 0 and 4. Because of this variation, the number with which each male started to develop could not be determined. However, the relatively low frequency of males with B's and the regular meiotic behavior of the latter suggested that most of the 105 males started with a single B. Cytological analysis of the cells of the gastric caeca of 31 males whose testes contained B's confirmed this suggestion by showing that only one male had two B's in these cells; all the rest had one. In the testes of the 74 other males the mean number of B's ranged from 0.89–2.50, but only two males had means higher than 2.00. The observed ratio of one male with two B's to 30 with one, suggested that only the two males with the highest means started to develop with two B's and that the other 72 males all started with one. Since the mean for the 72 males was 1.37 B's per male, it was concluded that during the development of the testes of these males the mean increased by 37%. The males with B's had fewer follicles in their testes and apparently had also a lower frequency of normal sperm. — The analysis of the testes of Locusta migratoria L. males from Japan gave results which agreed with those from C. pellucida.Supported by grants GB 1585 and GB 6745 from the National Science Foundation, Washington, D.C.     

Genotype-dependent effect of B-chromosomes on chiasma frequency in Eyprepocnemis plorans (Acrididae: Orthoptera)

In order to analyze the effect of B-chromosomes on chiasma frequency, the offspring of different females of the grasshopper Eyprepocnemis plorans have been studied. From the comparison between individuals of the different families having a given number of B's and between individuals with different numbers of B-chromosomes within the same family, it can be concluded that the effect of the genetic background on chiasma frequency is greater than that produced by the presence of B's, and that there is a between-families variation in the effect of B-chromosomes which, in addition, is negatively correlated with the chiasma frequency of related individuals without B's. This genotype-dependent effect of B-chromosomes is discussed.     

Cytogenetic studies on natural populations of grasshoppers with special reference to B-chromosomes

In Atractomorpha bedeli the frequencies of males with B-chromosomes in the males from the Tofuro-ato population were 21, 30, 32, 40, and 26%, respectively, over five years. Numbers of B's (0 to 7) in the primary spermatocytes were usually constant within the follicles but were variable among the follicles within the individuals. In males from the Okazyo-ato population, 35% contained B's in their primary spermatocytes. The cells of the gastric caeca were observed in 8 males with B's in the primary spermatocytes but no cells of the gastric caeca contained B's. Cells of ovariole walls and those of gastric caeca were analysed in 80 females from the same population; none of them contained B's. These facts indicate that B's are present in the germ line but absent in the somatic line. The difference between the germ line and the somatic one was explained on the basis of elimination of B's from the somatic cells. The inter-follicular variation in number of B's was ascribed to mitotic non-disjunction in the germ line prior to differentiation of the follicles.Contribution No. 86 from the Institute of Biology, Oita University.     

Replication of euchromatin and heterochromatin in a mealybug

Asynchronous DNA replication of euchromatic (E) and heterochromatic (H) chromosomes and heterochromatic B chromosomes (B) were studied in the mealybug, Pseudococcus obscurus Essig (Homoptera: Coccoidea). The study was carried out on mycetocytes of adult females and on spermatocytes of mid-second instar males by employing tritiated thymidine labeling and autoradiography. In the mycetocytes the incorporation of the labeled thymidine began and ended later in the B's than in the E chromosomes. The S period was found to be about 21 hours. The DNA replication of the E chromosomes occupied about 86% of the S period and that of the B's 33%; during 18% of the mid-S period the replication of the two types of chromosomes overlapped. In the meiotic S period of the spermatocytes, the DNA of the E chromosomes started to replicate earlier than that of the H chromosomes and the B's, but the replication of the E chromosomes, the H chromosomes, and the B's overlapped. The H chromosomes completed their replication much later than the E chromosomes and slightly later than the B's.Supported by grants GB 1585 and GB 6745 to Dr. Uzi Nur from the National Science Foundation, Washington, D. C.Part of a thesis submitted to the University of Rochester in partial fulfillment of the requirements for the degree of Doctor of Philosophy.     

B-chromosomes in some Himalayan hardwoods

B-chromosomes are reported for the first time for woody angiosperms. Out of six species from the eastern Himalayas in which B-chromosomes were observed, two are polyploid. The number of B-chromosomes varied from 0–5 in the different species. In Sambucus canadensis, Glerodendrum colebrookianum and Neolitsea zeylanica B's were found to pair with each other at Metaphase-I.The financial support for this work was provided by the U. S. Government from PL 480 funds in India (Grant: A7-FS-12).     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on chromosome pairing at meiosis was investigated in the species hybrid Lolium temulentum x L. perenne at both the diploid and tetraploid level. The presence of B chromosomes drastically reduced association of homoeologous chromosomes in both the diploids and tetraploids. This was evident from the high frequency of univalents recorded in PMC's of diploid hybrids with B's and from the predominantly bivalent association of homologous chromosomes in tetraploids of this type. In the absence of B's homoeologous pairing was extensive giving a high frequency of bivalents in the diploids and multivalents as well as bivalents and univalents in the tetraploids.     

The effect of B chromosomes on homoeologous pairing in species hybrids

Six diploid populations of Lolium rigidum were used in crosses with a standard homozygous line of Lolium temulentum in order to analyse the variation in the effect of B chromosomes on homoeologous pairing in the interspecific hybrid. — Large differences in chiasma frequency were found, both between the progeny of the different populations and, in one case, between the progeny of different plants from the same population. That this variation was due to differences in B chromosome genotype was ruled out by the use of appropriate 1B Lolium rigidum parents. It is concluded therefore that the effect of B's on homoeologous pairing in this interspecific hybrid can be modified quite substantially by the A chromosome genotype of the parents.     

The modification of crossing over in maize by extraneous chromosomal elements

Summary Five regions of the maize genome were tested for their response to endogenous factors influencing recombination. These included heterochromatic B chromosomes and abnormal chromosome 10 as well as the sex in which recombination occurred.The frequency of recombination in the proximal A ₂-Bt and Bt-Pr segments of chromosome 5 was increased in the presence of B chromosomes, with the male meiocytes showing a greater response than the female meiocytes. In addition, experiments involving 0, 1, 2 and 4 B's revealed a dosage effect of B chromosomes on crossing over in chromosome 5. Recombination in the proximal Wx-Gl ₁₅ interval of chromosome 9 was found to be slightly higher than normal in male flowers when two B chromosomes were present. This increase was accompanied by a decrease in the adjacent Sh-Wx segment. Crossing over in the distal C-Sh segment and in the C-Sh-Wx-Gl ₁₅ regions of female flowers was unaffected by B's.Comparisons of plants heterozygous for abnormal chromosome 10 (K10 k10) and homozygous for the standard chromosome 10 (k10 k10) showed that abnormal 10 greatly enhances crossing over in the A ₂-Bt and Bt-Pr segments of chromosome 5. In contrast to the finding with B's, the effect is greater in female than in male sporocytes. K10 showed no significant effect on recombination in the C-Sh-Wx-Gl ₁₅ region of chromosome 9 except in male sporocytes, where there was a slight increase in the Sh-Wx region of 0 B K10 k10 plants and a possible interaction with B chromosomes to raise the level of recombination between Wx and Gl ₁₅. The fact that the regions adjacent to the centromere of chromosome 9 show little or no response to the presence of K10 indicates that the proximal heterochromatin of this chromosome differs qualitatively from that of other maize chromosomes. This conclusion is supported by a comparison of the effects of B chromosomes, K10 and sex on crossing over in chromosomes 5 and 9.Dedicated to Dr. M. M. Rhoades on the occasion of his seventieth birthday.     

Supernumerary (B-) chromosomes in the Malayan house rat,Rattus rattus diardii (Rodentia,Muridae)

Three widely separated Malayan populations of Black rat, Rattus rattus diardii (Jentink), were found to comprise individuals with supernumerary (or B) chromosomes. The number of supernumeraries varied from 0 to 4. The incidence of B's in the three populations did not differ significantly. There was also no difference between the two sexes.     

Nuclear genetic activity in B-chromosome rye,in terms of the quantitative interrelationships between nuclear protein,nuclear RNA and histone

The nuclear basis of B-chromosome genetic activity in rye has been investigated using quantitative cytochemical techniques on isolated root tip nuclei. Nuclear DNA amount was found to be directly proportional to B-chromosome number. Relative amounts of total nuclear protein and nuclear RNA however, decreased with increasing numbers of B's but not in a strictly linear fashion. The values were disproportionately low for odd numbered B-classes of plants. Histone protein was found to increase as the number of B's went up, and in this case the values were disproportionately high for odd numbered B-classes. A negative correlation was found between histone and total nuclear protein and histone and nuclear RNA amounts.     

The B-chromosomes of Locusta migratoria I. Detection of negative correlation between mean chiasma frequency and the rate of accumulation of the B's; a reanalysis of the available data about the transmission of these B-chromosomes

B-chromosomes were studied in two Spanish populations of Locusta migratoria. Both exhibit a high frequency of B-carrying individuals (75% and 94%). In both the B-chromosomes are mitotically unstable and they accumulate in the male germ line. The mean rate of accumulation is 28.5% in Baños de la Encina and 31.9% in Carboneras. There are no significant differences in mean cell chiasma frequency and between-cell variance of chiasmata between follicles with different number of B's within individual males. Likewise, there are no significant differences in mean cell chiasma frequency and between-cell variance between males with and without supernumeraries. However, a significant negative correlation exists between mean chiasma frequency and the rate of accumulation.     

A cytogenetic analysis in Psophus stridulus (L.) (Orthoptera: Acrididae): B-chromosomes and abnormal spermatid nuclei

The male chromosome complement of Psophus stridulus (L.) (Orthoptera: Acrididae) has been analyzed by using orcein staining, C-banding and silver impregnation. During spermatogenesis only one pair of autosomes (M₉) shows an active nucleolar organizer region located in a C-banded constriction. There are other chromosome pairs with constrictions but these do not show nucleolar activity. The relationship between these constrictions and the C-banding pattern exhibited by this species is analyzed.In a sample of 83 males from five populations, two different supernumerary chromosomes were observed. Four males had a metacentric B-chromosome (B^m) similar in size to the sex chromosome and mitotically stable. Its meiotic behaviour indicates that it is an isochromosome. An additional small B-chromosome (B⁸) was also found in a single follicle of one individual carrying the B^m.A high rate of abnormal spermatids (macrospermatids) was scored in the individuals carrying B's. This proportion is notably higher in the follicle containing both the B^m and the B⁸.     

Transmission Comparisons of Cucumber Mosaic Virus Subgroup I and II Isolates by Different Aphid Species

To investigate the transmission differences between Cucumber mosaic virus (CMV) subgroup isolates, we carried out a comparative study with five aphid species Myzus persicae, Aphis gossypii, Lipaphis erysimi, Aphis craccivora and Megoura viciae in laboratory and field experiments to evaluate spread of CMV Subgroup I NX and subgroup II AG isolates in tobacco. Both NX and AG varied in transmission efficiency by the five aphids, and our transmission results revealed important differences in transmission efficiency of two isolates by Myzus persicae and Aphis gossypii. In contrast, significant transmission differences were not detected with Lipaphis erysimi, Aphis craccivora or Megoura viciae. Interestingly, the overall transmission efficiencies of the two different subgroup strains were almost equal when field transmissions were tested with mixed populations of the five aphid species. Our results together with our previously reported experiments on competition of CMV subgroup isolates in tobacco suggest that variations in aphid vector populations contribute substantially to the epidemic potential of CMV subgroup isolates.     

Nature of 2n gamete formation and mode of inheritance in interspecific hybrids of diploid Vaccinium darrowi and tetraploid V. corymbosum

RAPD markers were used to determine the level of heterozygosity transmitted via 2n gametes from V. darrowi selection Florida 4B (Fla 4B) to inter-specific hybrids with tetraploid V. corymbosum cv Bluecrop. The tetraploid hybrid US 75 was found to contain about 70% of Fla 4B's heterozygosity, a value attributed to a first-division restitution (FDR) mode of 2n gamete production. Crossovers during 2n gamete formation were evidenced by the absence of 16 dominant alleles of Fla 4B in US 75, and direct tests of segregation in a diploid population involving Fla 4B. RAPD markers that were present in both Fla 4B and US 75 were used to determine the mode of inheritance in a segregating population of US 75xV. corymbosum cv Bluetta. Thirty-one duplex loci were identified which segregated in a 51 ratio, indicating that US 75 undergoes tetrasomic inheritance.     

B-Chromosomes and E-1 isozyme activity in mosaic bulbs of Scilla autumnalis (Liliaceae)

The electrophoretic patterns of esterase (E-1), alcohol dehydrogenase (ADH), and glutamate oxaloacetate transaminase (GOT) isozymes were studied in two Spanish populations of the lily Scilla autumnalis with B-chromosome carrying individuals. The E-1 isozyme activity appears only in those individuals with B-chromosomes. None of the bulbs free of B's show it. Five bulbs, mosaic for B-content, were identified. Electrophoretic analysis shows that these bulbs are characterised by mosaicism for E-1 isozyme activity. An analysis of individual roots by both electrophoretic and cytological methods shows that tissue mosaicism for B-content correlates with tissue mosaicism for E-1 isozyme activity. The electrophoretic analysis of different roots from bulbs heterozygous for the Est-1 locus indicates that the structural gene for E-1 is not located on the B-chromosome itself. Rather there is a derepressor effect of Bs on E-1 isozyme activity. Since ADH and GOT patterns are unaffected by the presence of B-chromosomes it is clear that they do not exhibit a generalised derepressor effect.