Karyotype of Polyommatus (Agrodiaetus) eriwanensis Forster, 1960 and taxonomic position of P. (A.) interjectus de Lesse, 1960 (Lepidoptera, Lycaenidae)

The karyotype of Polyommatus (Agrodiaetus) eriwanensis Forster, 1960 from the type locality (“Eriwan” [Yerevan, Armenia]) and other localities in Armenia was investigated. The number of chromosomal elements (bivalents+ multivalents) observed in male meiosis I was found to vary from 29 to 34. In individuals with n = 34, all observed elements were represented by bivalents. In other specimens, heterozygosity for different number of chromosomal fusions resulted in multivalent formation at MI stage and consequently in a lower number of recognizable chromosomal elements. We show that all karyotype peculiarities of P. (A.) interjectus de Lesse, 1960 (n = 29–32) from Turkey are similar to those in A. eriwanensis. The butterflies of these taxa have allopatric distribution and can be considered as conspecific.

Two-Locus Local Adaptation by Additive or Epistatic Gene Combinations in Autotetraploids Versus Diploids

In this article, we present a theoretical comparison of local adaptation between diploid and autotetraploid populations when fitness is determined by either additive or epistatic interactions between alleles at 2 loci. A continent-island model of local adaptation is derived, with 1-way migration from the continent to the island and distinct genotypes adaptive on the continent versus the island. The meiotic component of the model accounts for multivalent formation and the processes of chromosomal gametic disequilibrium and double reduction, which are unique to autotetraploids. Both the adaptability and efficiency of adaptation are investigated, where adaptability asks whether a population adapts and efficiency is the rate of adaptation. With an additive genetic basis to fitness, diploids experience better adaptability and efficiency than autotetraploids. With epistasis, our results indicate a limited parameter space in which autotetraploids have greater adaptability than diploids, but results indicate an interesting difference between adaptability and efficiency of adaptation. Oftentimes, diploids exhibit greater adaptability whereas autotetraploids exhibit greater efficiency of adaptation. These findings provide evidence for the advantage of epistasis within autotetraploids when efficiency of adaptation is of interest. Although autotetraploids are more efficient, under the same conditions and at equilibrium, diploid populations often have higher mean local fitness. Overall, the most ideal situation for autotetraploid local adaptation compared to diploids is when epistasis is strong, mutation is weak, recombination is high, selection is strong, deleterious selection is additive, chromosomal gametic disequilibrium is present, and double reduction is absent.

Genotypic frequencies at equilibrium for polysomic inheritance under double-reduction

Polyploids are organisms whose genomes consist of more than two complete sets of chromosomes. Both autopolyploids and allopolyploids may display polysomic inheritance. A peculiarity of polysomic inheritance is multivalent formation during meiosis resulting in double-reduction, which occurs when sister chromatid fragments are segregated into the same gamete. Double-reduction can result in gametes carrying identical-by-descent alleles and slightly increasing homozygosity. This will cause the genotypic frequencies to deviate from expected values and will thus bias the results of standard population genetic analytical methods used in molecular ecology and selective breeding. In this study, we extend existing double-reduction models to account for any even level of ploidy, and derive the symbolic expressions for genotypic frequencies via two methods. Inbreeding coefficients and heterozygosity under double-reduction and inbreeding are also calculated. Numerical solutions obtained by computer simulations are compared with analytical solutions predicted by the model to validate the model.

Chromosome pairing affinity and quadrivalent formation in polyploids: do segmental allopolyploids exist?

When polyploid hybrids with closely related genomes are propagated by selfing or sib-breeding, the meiotic behaviour will turn into essentially autopolyploid behaviour as soon as the affinity between the genomes is sufficient to permit occasional homoeologous pairing. An allopolyploid will only be formed when the initial differentiation is sufficient to completely prevent homoeologous pairing (in some cases enhanced by specific genes), or when segregational dysgenesis prevents transmission of recombined chromosomes. A new polyploid hybrid may be considered a segmental allopolyploid and may show reduced multivalent formation as a result of preferential pairing between the least differentiated genomes. An established polyploid is either an autopolyploid or an allopolyploid. In exceptional cases it is thinkable that a stable segmental allopolyploid arises, in which some sets of chromosomes are well differentiated and behave as in an allopolyploid, whereas other sets are not well differentiated and behave as in an autopolyploid. No clear cases have been found in the literature so far. Key words : chromosome, pairing affinity, quadrivalent frequency, segmental allopolyploidy.

Cytogenetics of Lathyrus palustris, a natural autohexaploid

American as well as British forms of the perennial plant Lathyrus palustris have 2n = 42 chromosomes with one group of 6 long submetacentric chromosomes, two groups of 6 medium-long subacrocentric chromosomes, three groups of 6 medium-long to medium-short submetacentric to subacrocentric chromosomes, and one group of 6 medium-short metacentric chromosomes. One haploid complement measures 45.8 μm at somatic metaphase. At meiotic first metaphase both forms show extensive multivalent formation. In 50 metaphase I cells of one plant of the British form there were 51 ring hexavalents, 80 chain and open-branched hexavalents of various forms, 2 quinquivalents, 41 ring quadrivalents, 58 chain quadrivalents, 20 branched quadrivalents of various forms, 1 trivalent, 128 ring bivalents, 280 open bivalents, and 9 univalents. It is concluded that L. palustris is most likely a natural autohexaploid with frequent multivalent pairing, many points of pairing initiation, several partner exchanges per multivalent, and many interstitial chiasmata. It may well be the first fully documented true natural autohexaploid reported. Pollen fertility was 56.2% and seed set 37%, apparently sufficient for a perennial plant species.Key words: Lathyrus, natural, autohexaploid, meiosis, multivalents.

Cytogenetics of decaploid Agropyron elongatum (Elytrigia elongata) (2n = 70). I. Frequency of decavalent formation

The multivalents that appeared in the decaploid strain of Agropyron elongatum (2n = 10x = 70), a relative of wheat, ranged from trivalent to decavalent. Few univalents occurred. The metaphase I chromosome association in 12 cells where all configurations could clearly be identified averaged 0.42 ring X + 0.17 chain X + 0.42 ring VIII + 0.17 branched VIII + 0.25 chain VIII + 0.17 chain VII + 1.17 ring VI + 0.33 branched VI + 0.5 chain VI + 1.67 ring IV + 0.42 branched IV + 0.58 chain IV + 0.08 branched III + 0.17 chain III + 12.58 ring II + 3.75 open II + 0.25 I. The occurrence of decavalents, up to two in one cell, and of a cell with five multivalents, each of which involved more than five chromosomes, and many multivalents of ring shape indicated that the strain is autodecaploid.The chromosome associations of each cell can be interpreted as seven groups of 10 homologous chromosomes. The high frequency of bivalents indicated a tendency toward reduced multivalent formation, for which an explanation is suggested.Key words: Agropyron elongatum, meiotic configuration, decaploid, multivalent.

Multivalent formation and pairing behavior of germ line limited chromosomes in male meiosis of Acricotopus lucidus (Diptera, Chironomidae)

The pairing behavior of the germ line limited chromosomes of Acricotopus lucidus was investigated in male meiosis using G-banding. Each of the nine different types of limited chromosomes can be absent or can be present in metaphase I with two or four homologous chromosomes, one type even with up to 10. Usually the homologues form bivalents, but frequently quadrivalents and also hexavalents consisting of the same chromosome type were observed. In some cases multivalents composed of different limited chromosomes occurred. This resulted probably from pairing and crossing-over between hom(oe)ologous segments in otherwise nonhomologous chromosomes. The observations are discussed in relation to origin and diversity of the germ line limited chromosomes.Key words: germ line limited chromosomes, multivalent formation, male meiosis, Acricotopus lucidus.

Cytogenetic analyses of intersectional hybrids between Parthenium argentatum and Parthenium confertum

Induced tetraploid guayule (Parthenium argentatum Gray, a rubber-producing and woody species of section Parthenicaeta) was crossed to P. confertum var. lyratum Gray, a herbaceous species of section Argyrocheta. Hybrids were obtained when guayule was used as female. Like their diploid progenitors, induced tetraploids were sexual and self-incompatible. At meiosis, they averaged 0.30 I, 29.63 II, 0.12 III, and 3.02 IV. Parthenium confertum had 69 chromosomes and was apomictic. At meiosis, it averaged 1.00 I, 32.98 II, and 0.51 IV. Hybrids received 36 chromosomes from guayule and 34 chromosomes from P. confertum. At meiosis, they averaged 0.52 I, 33.44 II, 0.20 III, and 0.50 IV. F1 plants behaved like an amphidiploid in which homologous chromosomes of guayule genome paired with each other and homologous chromosomes of P. confertum genome also paired preferentially. Chromosome association in the hybrids indicated that P. confertum parent may be a tetraploid with suppressed multivalent formation. Although hybrids were partially fertile and produced backcross progeny, the transfer of desirable traits (rapid growth and herbaceous growth habit) from P. confertum into guayule will depend on elimination of preferential pairing in the hybrids.Key words: guayule, natural rubber, meiosis, chromosome pairing.

Elimination of multivalents during meiotic prophase in Scilla autumnalis. II. Tetraploid

The pairing behaviour of chromosomes during meiotic prophase in two Scilla autumnalis tetraploids was investigated by observing synaptonemal complexes from serial sections and whole-mount surface spreads. In one of the tetraploids with the genomic constitution AAB7B7 pairing at first metaphase of meiosis is restricted to homologous chromosomes and only bivalents appear. In the second tetraploid, AB7B7B7, some multivalents are found at first metaphase, but most of the configurations are bivalent. The bivalents, however, result from chiasma formation between homoeologous as well as homologous chromosomes. Both tetraploids show extensive multivalent formation at zygotene. The transformation of these multivalents to bivalents is described and interpreted in the two tetraploid forms.Key words: Scilla autumnalis, synaptonemal complex, multivalents, elimination, polyploids.

Hierarchical control of multivalent formation in natural populations of a hexaploid perennial grass ( Koeleria vallesiana )

Observations on variation in multivalent frequency in Koeleria vallesiana are reported. The species is self-incompatible and has a standard karyotype divided into six haploid genomes each of four metacentric and three submetacentric euchromatic chromosomes (2.6–3.6 μm), one of the submetacentrics being a nucleolar–organizer with a minute satellite on the short arm. Of the 83 plants examined, 67 had the hexaploid number 2 n = 6 x = 42,12 were aneuploids (2 n = 41-45) and four had a single B-chromosome. At first metaphase of meiosis 92% of chromosomes form bivalents. Quadrivalents are the most common multivalents (5.5%) and sexivalents (1.1 %), the largest. Each chromosome of the standard complement is equally likely to be incorporated in a multivalent. There is extensive variation in chiasma number per pollen mother cell between cells (21–43) and plants (22.6–38.8) but not between populations (27.6–31.5) or countries (29.1–30.1). Variation in multivalent frequency is partly chiasma-dependent and partly synaptic in origin. It is not associated with the variation in chromosome number. Phenotypic variation at synapsis influences the frequency of pairing partner exchange and is found both between plants and between populations. Some populations are more variable in synaptic behaviour than others. The mechanisms determining such synaptic variation are unknown but their investigation should provide a useful insight into the general nature of chromosome homology as well as shedding light on evolutionary processes in polyploids.