SEXUAL AND APOMICTIC REPRODUCTION IN CALAMAGROSTIS (GRAMINEAE) FROM EASTERN NORTH AMERICA

Reproductive mode and chromosome numbers were determined for populations of several species of Calamagrostis from eastern North America. Calamagrostis pickeringii (2n = 28), C. perplexa (2n = 70), C. porteri subsp. porteri (2n = ca. 88–100) and C. porteri subsp. insperata (2n = 56) all have a sexual pattern of megagametophyte formation; the basal megaspore of a tetrad develops into a Polygonum-type embryo sac with proliferating antipodal cells characteristic of the Gramineae. In these four taxa self-incompatability, population structure and infrequent flowering limit seed production; they persist primarily by rhizomes and occupy relatively stable, late-successional habitats. Calamagrostis stricta subsp. inexpansa includes apomictic variants (2n = ca. 104–123)that produce megagametophytes by diplospory; the single archesporial cell divides mitotically to produce an embryo sac appearing identical to those formed by sexual species. The embryo and endosperm develop autonomously from egg cell and polar nuclei, respectively. Some apomictic individuals occasionally produce some pollen and may have the potential for reproducing sexually. Their seed set insured by apomixis, variants of subsp. inexpansa colonize disturbed, open habitats and have achieved wide distributions in glaciated regions of North America. Reinterpretation of relationships among taxa I examined necessitates the following new nomenclatural combinations; C. porteri subsp. insperata (Swallen) comb. nov. is based on C. insperata Swallen; C. stricta subsp. inexpansa (A. Gray) comb. nov. is based on C. inexpansa A. Gray and includes C. lacustris (Kearney) Nash and C. fernaldii Louis-Marie.