DISPERSAL AND GENETIC STRUCTURE IN KANGAROO RATS

We used spatial autocorrelation of allele frequencies to examine local structure in a population of bannertailed kangaroo rats for which Wright's isolation-by-distance model seems applicable, and for which we can estimate neighborhood size based on 10 years of data on demography and dispersal. The uniform dispersion and strong philopatric tendencies of this species provide a test case for the idea that restricted dispersal can lead to local genetic structure in small mammals. Whether we considered such complications as nonnormal dispersal distances, variation in lifetime reproductive success, fluctuating population density, and adult as well as juvenile dispersal, our estimate of effective population size was fewer than 15 animals. Nevertheless, data from four polymorphic allozyme loci analyzed over a range of separations between 50 m (approximately one home range diameter) and 1,000 m detected no evidence for spatial clustering of alleles. One resolution of this apparent paradox is that “gamete dispersal,” caused by the movements of males away from their residences during the breeding season, may be a significant (and unmeasured) component of gene dispersal. Our analyses also demonstrate that a decline in population density may actually increase neighborhood size. A more general implication is that even extremely philopatric mammals have effective population sizes large enough to prevent the development of local genetic structure.     

SONG POPULATIONS AND DISPERSAL IN STEELBLUE AND PURPLE WIDOWFINCHES

Payne, R. B. 1985. Song populations and dispersal in Steelblue and Purple Widowfinches. Ostrich 56:135-146.

Songs of Steelblue Widowfinches Vidua chalybeata were recorded over eight years at Lochinvar National Park, Zambia. Neighbouring males share their song-type repertoires. Individual birds changed the structural details of all song types in their repertoire from year to year, and over 4–5 years the songs accumulated changes so that a song type could scarcely be recognized as the same. A few males in addition switched their song repertoires when they dispersed from one song population to another, or when their old neighbours disappeared and new neighbouring males sang a different song repertoire. Individual marked adult birds moved from one song neighbourhood to another. Purple Widowfinches had a similar song behaviour but only one song dialect was found in the study area.

An estimate of the proportion of immigrants was made from the proportion of birds with songs unlike their neighbours, and from birds that had a mixed song repertoire or that switched their song repertoires. Song populations were sampled at several localities in the Transvaal, Botswana, Zambia, Kenya, Tanzania, Cameroon, and Nigeria. An overall minimal estimate of dispersal between song neighbourhoods was 18 %. The incidence of observed dispersal and the movements indicated by the song differences among local widowfinches were considerable and more than sufficient to prevent genetic isoloation and differentiation of local song populations.     

DIASPORE MORPHOLOGY AND SEED DISPERSAL IN SEVERAL WIND-DISPERSED ASTERACEAE

I made measurements of morphology and settling velocity on seeds of 19 species of wind-dispersed Asteraceae. From the morphological measurements I calculated Reynolds numbers and approximate plume loadings for the species. Diaspore settling velocity increases linearly with the square root of plume loading. This relationship varies among species and among subfamilies, but not among life history types. Reynolds number is highly variable among subfamilies, less so within subfamilies. Diaspores with beaked achenes have significantly lower settling velocities than diaspores with unbeaked achenes, even though beaked and unbeaked achenes do not differ in plume loading or in Reynolds number. Reynolds numbers of all diaspores examined are well above the range in which Stokes' Law applies. I recommend that the use of formulae based on Stokes' Law be curtailed in studies of the relationship between plume loading and settling velocity. The results suggest that many seed characters may have evolved due to selection on dispersal ability. This is in spite of phyletic constraints on morphology reflected in the relative uniformity of Reynolds numbers within subfamilies.     

DIMENSIONAL CORRELATIONS IN DEVELOPING SELAGINELLA SPORANGIA

With length of sporangia as a developmental index, the growth relationships of sporangia during differentiation were studied in strobili of Selaginella bigelovii. The strobili usually contain two rows of megasporangia and two rows of microsporangia with a mega- opposite a microsporangium at each node. Prior to the sporocyte stage a sporangium in a megasporangiate row is larger and elongates more rapidly than a sporangium opposite it at the same node in a microsporangiate row. The number of sporogenous cells is similar in sporangia of the same length from both rows until cell multiplication ceases in sporangia of the megasporangiate row, while it continues in sporangia of the same size in the microsporangiate row. The observed growth differences between sporangia of the micro- and megasporangiate rows are interpreted as events in the differentiation of two sporangial types.     

VARIATIONS IN TIGHT AND GAP JUNCTIONS IN MAMMALIAN TISSUES

The fine structure and distribution of tight (zonula occludens) and gap junctions in epithelia of the rat pancreas, liver, adrenal cortex, epididymis, and duodenum, and in smooth muscle were examined in paraformaldehyde-glutaraldehyde-fixed, tracer-permeated (K-pyroantimonate and lanthanum), and freeze-fractured tissue preparations. While many pentalaminar and septilaminar foci seen in thin-section and tracer preparations can be recognized as corresponding to well-characterized freeze-fracture images of tight and gap junction membrane modifications, many others cannot be unequivocally categorized—nor can all freeze-etched aggregates of membrane particles. Generally, epithelia of exocrine glands (pancreas and liver) have moderate-sized tight junctions and large gap junctions, with many of their gap junctions basal to the junctional complex. In contrast, the adrenal cortex, a ductless gland, may not have a tight junction but does possess large gap junctions. Mucosal epithelia (epididymis and intestine) have extensive tight junctions, but their gap junctions are not as well developed as those of glandular tissue. Smooth muscle contains numerous small gap junctions The incidence, size, and configuration of the junctions we observed correlate well with the known functions of the junctions and of the tissues where they are found.     

SEED DISPERSAL IN VIOLA (VIOLACEAE): ADAPTATIONS AND STRATEGIES

Surveys of Viola dispersal mechanisms result in the distinction of two major adaptive syndromes: one purely myrmecochorous for the exploitation of seed-transporting ants, the other partially myrmecochorous (= diplochorous) for the explosive ejection of seeds followed by ant exploitation. Diplochory is exhibited by the majority of Viola species, but myrmecochory is exhibited only by eleven species with limited Eurasian distribution. Morphological and experimental evidence suggests the hypothesis that Viola is basically myrmecochorous but that different selective pressures, especially seed predation, have produced a clear divergence in dispersal systems. The majority of species, the diplochores, have evolved a system which combines ballistic and ant seed dispersal with predator avoidance. Diplochory itself may be a response to predation pressure. The minority of species are purely myrmecochorous, possibly highly coevolved with specific ant species, thus limiting the distribution of the Viola species concerned.     

VEGETATIVE DISPERSAL IN OXALIS CERNUA

Vegetative propagation and vegetative dispersal of Oxalis cernua were studied. O. cernua is a bulbous dicotyledonous plant. The shoot of a grown plant is divided into two distinct parts: a vertical stem growing upwards and bearing the feeding roots, and a rootless horizontal thin stem. This last is pulled by means of a large contractile root and carries along with it a series of small buds, which form new bulbs at the end of the season. Several new bulbs are formed along the vertical shoot as well. Thus, the propagation bulbs of O. cernua disperse along two axes, which are at right angles to each other.     

DEVELOPMENTAL STUDY OF BRANCHED RHIZOPHORES IN THREE SELAGINELLA SPECIES

A morphogenetic investigation was made of the rhizophore of three large-sized tropical Selaginella species. The rhizophores of Selaginella delicatula, S. caudata, and S. plana arise exogenously at the points of branching of the main stems. In S. delicatula they are initiated at the junction of the second youngest branching. The rhizophore apical meristem has a tetrahedral apical cell and is capless. The rhizophores are usually three or four times dichotomously branched in S. delicatula and S. plana and four or five times in S. caudata. In S. delicatula, dichotomous branching of the rhizophore involves formation of two new apical cells subsequent to loss of an original apical cell. A pair of roots is formed endogenously from inner cells below the dermal layer at the apex of ultimate rhizophore branches. The finding that the rhizophore is an autonomously branched, leafless, and capless axis leads us to argue that Selaginella rhizophores, like lepidodendrid rhizomorphs, are fundamental axial organs that coordinate with the stem and root.