Chromosomal polymorphism in Pissodes weevils: further on incompatibility in P. terminalis

CHROMOSOMAL POLYMORPHISM AND INTER-RELATIONSHIPS IN PISSODES WEEVILS: ADDITIONAL CYTOGENETIC EVIDENCE OF SYNONYMY

Chromosome comparison and crossability tests compel relegating 6 of 10 Hopkins' Pissodes taxa to synonymy, namely, utahensis with similis Hopk., nigrae and alascensis with rotundatus Lec., fraseri and piperi with dubius Rand., and curriei with affinis Rand. Two, dubius and rotundatus, and possibly a third, affinis, of the four valid species revealed meiotic markers of semi-incompatibility not previously encountered; presumably, as in the strobi complex, the fourth, similis, is merely devoid of the necessary chromosomal diagnostic.

MEIOSIS IN DIABROTICA (COLEOPTERA: CHRYSOMELIDAE): CHIASMA FREQUENCY AND VARIATION

Fifteen species of Diabrotica have been examined cytologically. Except for the variable presence of supernumeraries in three of the species, the karyotype is uniform throughout, with 18 meta- or submetacentric autosomes and an XX ♂: XO ♀ sex-determining mechanism. Chiasma frequency is quite variable; in addition to the basic nine per cell, formation of a second chiasma (in the other arm) results in ring frequencies ranging from zero to nine per cell. Analysis of chiasma frequency and distribution at diakinesis and metaphase reveals that the rings are bi-chiasmate and not the result of ectopic pairing. With this in mind, chiasma formation is discussed in terms of interference. Intra-arm interference is complete; the upper limit of chiasmata per arm is invariably one. The strength of interference across the centromere determines the probability of formation of a chiasma in the other arm. When interference is strong, a second chiasma is rarely formed; as interference weakens, a chiasma is more easily formed in the other arm, resulting in a higher chiasma frequency. Associated with decreasing interference and an elevated chiasma frequency is an increase in cell variance; chiasma formation is less rigorously controlled, and variability in the number of rings per cell increases. The variability of ring formation in the genus militates against distinguishing taxa on this basis. However, the importance of increased chiasma frequency (coupled as it is with greater variance) resides in its influence on the genetic plasticity and adaptive potential of the species.

CHROMOSOMAL POLYMORPHISM IN NORTH AMERICAN PISSODES WEEVILS: STRUCTURAL ISOMERISM

Pissodes approximatus and P. canadensis, two taxa named by Hopkins, are shown to constitute a hybrid complex that forms a numerical-morphological karyocline between the southeast of North America and the Yukon. The two A metacentrics and the two B submetacentrics in the SE (predominantly 2n = 30) are each sequentially replaced by twin acrocentrics, aa and bb, in the NW (mainly 2n = 34). The two b pairs, of markedly different length, are segmentally interchanged in the SE relative to those in the NW, the distal interchanged segments being equal. The two Bs, derived by fusion of alternative acrocentrics, simulate pericentrically inverted, complementary submetacentrics. Introgression of the four structurally different types, commonest at the centre of the karyocline, results in 10 distinct B: bb karyotypes of which only 6 appear different; the other 4 are mirror images, or isomers.The intermediate karyotypes have been synthesized by crossing different AB and aabb taxa; they implicate two pairs of isomeric species in the origin of the polymorphic complex. Of these, one AB species with 'standard' Bs and two aabb species with relatively interchanged bs are extant. The fourth, a species with inverted Bs, may be extinct, have merely escaped detection, or else is completely introgressed.Chiasma formation is near perfect in natural polyisomorphs and almost equally so in synthetics. In fact, in both bb/bb heterozygotes the maximum number of associations per cell, normally 16 (the XY pair is achiasmate), is increased by the formation of anomalous quadruple configurations resulting from extra unions at the centric ends of the bs. If chiasmate and situated in minute arms, these unions far exceed expectation on a length-frequency basis, for such arms cannot be resolved in colchicined gonial metaphases. Whether the extra unions are truly chiasmate, as believed, or not, they nevertheless greatly influence the first metaphase disjunctional arrangement of the multiples and cause production of about 80% unbalanced gametes vs. the 50% expected of random assortment, thus generating a genetic isolating mechanism. Such acrocentric heterozygotes, though carrying a heavy segregational-sterility load, persist by recurrent SE-NW introgressive hybridization.