Facilitate Gene Flow Research Articles

DYNAMICS OF A NOVEL CHROMOSOMAL POLYMORPHISM WITHIN A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE SCELOPORUS GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE).

Several chromosome races of the mesquite lizard, Sceloporus grammicus complex, hybridize at localities in central Mexico. In most cases, the hybridizing populations are delineated by centric fissions at one or more of the macrochromosomes. One notable exception is the Tulancingo hybrid zone between the F5 and FM2 cytotypes. In addition to fission and/or inversion differences at chromosomes 1, 3, 4, and 6, these races differ by a complex rearrangement of chromosome 2, which carries the nucleolus-organizer region in this species. The meiotic consequences of heterozygosity at this chromosome were examined in males to assess the potential for this chromosome to contribute to the dynamics of the hybrid zone. Chromosomal analysis revealed several putative F1 hybrids and confirmed the production of nonparental chromosomal morphologies through recombination. Pachytene analysis revealed meiotic pairing difficulties associated with chromosome 2 in males heterozygous for the parental chromosomal morphologies. Significant aneuploidy is expected because of random disjunction of the chromosome-2 elements. As a result, these males likely suffer reduced fertiliity and fitness. In contrast, males heterozygous for recombinant chromosomal morphologies displayed low levels of meiotic irregularities and presumably exhibit higher fertility than individuals heterozygous for parental morphologies. It is hypothesized that the recombinant phenotypes facilitate gene flow between the F5 and FM2 cytotypes.

GENETIC DIFFERENTIATION OF POLISH POPULATIONS OF SOREX ARANEUS L. III. INTERCHROMOSOMAL RECOMBINATION IN A HYBRID ZONE.

Two parapatric chromosomal races of the common shrew (Sorex araneus) in Poland differ in their complement of metacentric arm combinations: hk, io, gr, nm (race IV), and hi, ko, gm, np (race II). In hybrids, these eight race-diagnostic metacentrics form two randomly segregating complexes. The first complex (C1 ) occurs in the form of a ring configuration ok/kh/hi/io, or a chain o/ok/kh/hi/i (when there is Robertsonian polymorphism of the element io). The second complex (C2 ) always takes the form of a six-element chain configuration r/rg/gm/mn/np/p. The C2 complex may be shortened to five or even four elements, when acrocentrics g, m and n are present. In the contact zone we found shrews of pure races (race II or IV), as well as hybrids with C1 or C2 complexes, and recombinants hi, ko, gr, nm. Complex heterozygotes are likely to suffer reduced fertility due to malsegregation at meiosis. However, the C1 hybrids with ring configurations occur with a high frequency throughout the contact zone. This suggest that their fitness is only slightly lowered relative to pure race individuals, in contrast to the hybrids with C1 or C2 chain configurations, which presumably have a more heavily reduced fertility. On the other hand, at the center of the zone there is a high proportion of recombinants, which, being chromosomal homozygotes, should display normal meiotic segregation. Furthermore, the high frequencies of recombinants within the contact zone should facilitate gene flow between the races. The occurrence of recombinants plays a similar role as the appearance of the maximum frequencies of acrocentric homozygotes described in several contact zones of S. araneus.

Genic Variability, Dispersal, and Differentiation in Two Species of Ophiuroids from Southern California

Genic Variability, Dispersal, and Differentiation in Two Species of Ophiuroids from Southern California

Nature reserve designation in a cultural landscape, incorporating island biogeography theory

Nature reserve designation has traditionally been based on physiographic and botanical considerations in conjunction with administrative and practical realities. Considerations derived from the theories of island biogeography and population genetics have significant bearing on the maintenance of species. These are not usually incorporated into the designation process. The conceptual model presented here incorporates such concerns in a system that integrates geological, botanical and zoological features. The model also recognizes the importance of value judgments, objectives and administrative concerns. The model applies primarily to cultural landscapes and can be implemented at a variety of scales from the international to the local level. It incorporates a number of important concepts such as the possibility of food chain and seral stage manipulation to achieve a desired community or trophic structure. Linkage systems to facilitate gene flow among reserves are also discussed. This comprehensive approach to nature reserve designation can contribute significantly to the rational selection of reserve sites, and thus to the conservation of flora, fauna and geological formations.

Insertion, excision, and inversion of Tn5.

The defining characteristic of bacterial transposons is their ability to move to new loci in the absence of extensive DNA sequence homology or the recA protein (for review, see Calos and Miller 1980; Starlinger 1980). Transposons increase the fitness of bacterial populations by facilitating gene flow among different species and by changing the arrangement and the control of expression of genes within a single species. Transposons that contain genes for antibiotic resistance appear to be responsible for the recurrence of the same resistance genes in otherwise unrelated R-factor plasmids (see Falkow 1975). In the laboratory, such resistance transposons can be used to mutate host genes, to probe and alter the control of gene expression, and to move genes to new locations.

Dispersal in Small Mammals

It is not surprising that dispersal of individuals or propagules at some stage of the life cycle is a characteristic of most biological species. An organism requires some finite space in the environment from which resources neces­ sary for its survival are acquired. The size of this space may fluctuate with changes in resource availability and population density, but there exists some critical minimum space that must be maintained if the organism is to persist. Although resources within this space may be diverted into repro­ duction and the rearing of young, the continued occupancy of the parent's resource space by the offspring, or vice versa, is a threat to survival. Yet the movement of either parent or offspring away from the natal site is likely to place the disperser at risk. The dispersal of individuals can have profound effects on the genetic structure and demographic processes of a population. The movement of individuals between populations facilitates gene flow and the maintenance of genetic variability (56, 76). Since dispersal promotes the utilization of new or depopulated areas, it can facilitate the persistence of local popula­ tions and therefore the stability of the species over large geographic areas (136, 139). In spite of this, ecologists until recently have largely neglected dispersal as a demographic parameter that affects population density. This may be because we cannot distinguish between natality and immigration or