Spatial Pattern and Gamete Dispersal Distances in Atrichum angustatum, a Dioicous Moss
1977; American Bryological and Lichenological Society; Volume: 80; Issue: 2 Linguagem: Inglês
10.2307/3242472
ISSN1938-4378
Autores Tópico(s)Lichen and fungal ecology
ResumoStrong segregation of sexes in populations of Atrichum angustatum (Brid.) B.S.G. is attributed to vegetative proliferation of unisexual clones. The distribution of gamete dispersal distances in this moss is highly leptokurtic, with mean of 2.11 cm and an observed maximum of 11.0 cm. Overall shoot density is 4.6 per cm2, giving neighborhood size of about 225 individuals and neighborhood area of about .005 m2. When leafy vegetative gametophores are dissected and classified as to sex, the overall sex ratio approaches unity. These observations are compared to previous studies of sporophyte production in relation to population sex ratio and relative spatial distribution of males and females. It is concluded that sex ratio and spatial pattern to large degree determine levels of sporophyte production in dioicous moss populations. The few data currently available for dioicous mosses regarding spatial pattern of male and female plants and gamete dispersal distances are intriguing, but much less than definitive. Quantitative field studies of the ratio and distribution of male and female gametophores are lacking. Gamete dispersal distances are likewise known only from reports of absolute maximum and minimum distances. Such information is important in any discussion of the population biology of mosses. The present study was undertaken to cast some light on these basic questions. Newton (1971) has recently reviewed the bryological literature with reference to sex ratios in moss populations. In studies of Mnium undulatum Hedw. an approximate 4:1 bias in favor of females was discovered (Lacey, 1950; DeSloover, 1966; Newton, 1971). Similar results were obtained for Climacium dendroides Web. & Mohr (Bedford, 1938b) and Breutelia arcuata Schimp. (Bedford, 1940). Tallis (1957), however, found an excess of male gametophores in his analysis of Rhacomitrium lanuginosum (Hedw.) Brid. The study of sex ratios in bryophytes is complicated by the presence of nonreproductive shoots. Riemann (1972) scored 10,000 herbarium specimens of Rhytidiadelphus triquetrus (Hedw.) Warnst., of which 49% were female, 33% were male and 18% were apparently sterile. Using cytological markers to distinguish sexes, Newton (1971) found that the frequency of production of antheridia underestimates the actual number of male gametophytes. Both large-scale and small-scale distribution patterns and ratios of male and female 1 I thank Lewis E. Anderson and Ann H. Stoneburner for encouragement, advice and criticism. They, with Janis Antonovics, Norton G. Miller and Maxine A. Watson, commented on an earlier draft of the manuscript. 2 Department of Botany, Duke University, Durham, North Carolina 27706. Present address: Department of Biology, Texas AM 1938b), Breutelia arcuata (Bedford, 1940), Aulacomnium androgynum (Hedw.) Schwaegr. (Persson, 1940), Leucobryum glaucum (Hedw.) Schimp. (Persson, 1943), Mnium undulatum (Lacey, 1950), Bryoxiphium norvegicum (Brid.) Mitt. (Hagne & Welch, 1951), Pleurozium schreberi (Brid.) Mitt. (Longton & Greene, 1969), Rhytidiadelphus triquetrus (Riemann, 1972) and several island species of eastern Asia (Iwatsuki, 1972). Quantitative tests of the assumption that gene flow in terms of gamete dispersal is over very short distances in moss populations have been few. More typical are qualitative observations like those of Harvey-Gibson and Miller-Brown (1927) that mites, springtails and other arthropods that feed on the paraphyses of Polytrichum commune Hedw. frequently transport sperms a considerable distance. Goebel (1930; p. 968) and Bower (1935; pp. 82-83) concluded that aquatic mosses produce sporophytes only rarely due to dilution of sperms. Bedford (1938b) never found female plants bearing sporophytes more than three inches from male in Climacium dendroides. In Breutelia arcuata, which has cup mechanism for dispersal of sperms, the distance between sporophyte-bearing plants and male gametophores never exceeds one inch (Bedford, 1940). Brodie (1951) was able to experimentally disperse sperms up to two feet from the splash cups of Polytrichum ohioense Ren. & Card. Longton and Greene (1969) estimated the fertilization range of Pleurozium schreberi to be 7 to 10 cm, and for Mnium undulatum distances range from 2.5 to 12 cm with maximum of 20 cm (Newton, 1971). Riemann's (1972) populations of Rhytidiadelphus triquetrus showed high sporophyte production when the mean distance between sexes was less than 3 cm and no sporophyte production at distances greater than 6 cm. In all of these studies minimum estimates of gene flow distances were obtained, since the distance from sporophyte to the nearest male gametophyte obviously tends to underestimate the actual distances traveled by sperms. The only data regarding gamete dispersal in monoicous mosses is the study of Weissia controversa Hedw. by Anderson and Lemmon (1974) in which the average gene flow distance was measured to be 12.3 mm and the median, slightly less than 5 mm. The maximum distance observed was 40 mm. Atrichum angustatum (Brid.) B.S.G. is common dioicous moss of disturbed sites in oak-hickory forests of the eastern United States (Nyholm, 1971; Crum, 1973). In the spring and early summer leafy male gametophytes produce antheridia within perigonial inflorescences that are quite distinct from the inflorescences of females. The growth habit of A. angustatum is similar to other dioicous acrocarpous mosses, and dense, pure stands are often developed. These characteristics facilitate mapping and measurement of distances between plants. The plants are perennial and may produce additional antheridia on leafy shoots extending from the previous year's perigonium. Atrichum therefore, like Polytrichum, is one of the few moss genera in which production of antheridia does not use up the apical cell (Watson, 1971; p. 112). These observations are currently under further investigation by monitoring marked individual leafy gametophytes. This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:08:01 UTC All use subject to http://about.jstor.org/terms I I I I I I I I I I I I I I 0-0000000000-00 000000 0 000000000000 0000 00 0 000-0000000000 0000 00000000-0o----000-000000000-0000-o000000000000000000000000000000000000000000000000000000000000000-00--000o00 -*-oo0 o o o oo---000000000 00000000000000000000000000000 -00000000000000000000000000000-0000000 0 *000000000000000000-0-o000000-0000-0-000--0000000o-0 ---om0000-0000 000-00o-o--mo-@ @ @ * 0000-00 00-0--0 00000000000000000000000000000000000000 000 00--00000000-00--000-0-0-0 0 0000-o0000000000o -0-o--00 e000000--0 00000000000--00-0000-00000*0000000000000000000000000000000o0000-000o0-000-0-00000-00oo----o000 0000 -00 00 -0--00000-00000 00ooooooooooo'ooo -oo00 o--oooooooooooooo ---00-0-00-0000---0-----0-0000 --000000000 000 00000000 --00000--00-00000000000 000 000 FIGURE 1. Line transects in the Atrichum angustatum study population in the Duke Forest, Orange County, North Carolina. Each division marks 10 cm. Dashes represent male gametophores with perigonia; open circles, gametophores of undetermined sex; and solid circles, female gametophores with sporophytes. The longer axis from left to right is oriented north-northeast. L,-I 00
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