CHROMOSOMES OF CRUCIFERAE I. DESCURAINIA

Abstract

As UNDERSTOOD by Schulz (1924), Descurainia Webb & Berthelot is a natural assemblage of 43 species of annual and biennial cruciferous plants. Dething (1939) revised North American species of genus. Interpreting species broadly on morphologic, ecologic, and geographic evidence, he regards the subspecies as product of selective action of a particular environment and, accordingly, recognized for Descurainia North America 9 native and I adventive species. (Fernald [1940] claims that Detling, at least some instances, incorrectly used term subspecies in sense of long-established term varietas.) Southwestern United States is distributional center of native representatives of genus. Dr. LeRov E. Detling sent seeds collected from wild plants of four species and ten subspecies of Descurainia. Plants from those seeds were grown Botanical Gardens of University of Michigan. Chromosomes of plants were counted at metaphase leaf smears prepared by method of Baldwin (1939). CHROMOSOMES AND THE PLANTS.-The chromosomes of Descurainia are uniformly small. The chromosome numbers form, with exception of one report, a regular polyploid series founded on 7. Previous workers have published chromosome numbers for four species of genus. Two of those species are South American: D. Cumtngiana (Fisch & Meyer) Prantl, with 14 somatic (Manton, 1932), 7 gametic (Jaretzky, 1932) chromosomes; D. myriophylla (Willd.) Fries, with 14 and, some cells, 28 somatic chromosomes (Manton, 1932). The North American representatives of Descurainia for which chromosome numbers are known are given below as interpreted systematically by Detling. Data on geographic occurrence are taken from that author. Detling's herbarlum numbers and places of collection are put brackets. D. Sophia (L.) Webb, type species, is an annual or biennial plant of Old World and is now distributed throughout United States, Canada, and Alaska. Manton (1932) reported for species 28 and, some cells, 56 somatic chromosomes. Jaretzky (1932) found 14 gametic chromosomes, Mayor (1934) emphasized difficulty of counting chromosomes of this plant and provisionally reported 10 gametic, 20 somatic chromosomes for species, designated by him Sisymbrium Sophia L. The present writers determined a 2n-number of 28 for species [IDetling 2217, Mountain Home, Idaho] (fig. 1). D. Richardsonii (Sweet) Schulz is biennial andoccurs, for most part, mountains of western Unlted States; one subspecies is northern plains. Two collections of subspecies vis' Received for publication July 17, 1940. Papers from Department of Botany of University of Michigan, No. 723. cosa (Rydb.) Detl. [Detling 2257, Targhee Pass, Idaho; Detling 2213, Hilgard, Oregon] were found dulring this study to have 14 somatic chromosomes (fig. 2), subspecies procera (Greene) Detl. [Detling 2263, Howard Springs, Targhee Pass, Idaho) to have a 2n-number of 28 (fig. 3), and subspecies incisa (Engelm.) Detl. [Detling 2380, Flagstaff, Arizona], a 2n-number of 42 (fig. 4). It is clear that these subspecies are separated by polyploid differences. Consideratlon of ranges given by Detling shows that diploid viscosa and hexaploid incisa occupy geographic areas of approximately same magnitude, latter subspecies being more southern its occurrence but extending into range of viscosa; and that, of these three subspecies, tetraploid procera has most restricted range and occurs almost entirely within region where both incisa and viscosa are found. D. obtusa (Greene) Schulz is an aggregate of strict, coarse, biennial plants mostly mountains or on high plateaus of southwestern United States and northern Mexico. The present authors found that subspecies typica Detl. [Detling 2381 Williams, Arizona] has 2n=14 (fig. 5), and that subspecies brevisiliqua Detl. [Detling 2375, Gonzales, New Mexico, type collectionDetling 2374, Thoreau, New Mexico] has 2n=42 (fig. 6). The diploid typica has a greater geographic range than hexaploid brevisiliqua. Most of range of diploid is south of that of hexaploid. D. pinnata (Walt.) Britt. is a group of annual plants. Evolution within this species seems to have followed two distinct lines, giving rise to one complex of subspecies hot arid regions of Southwest and another complex more northern cooler and frequently moister regions of Rocky Mountains, Pacific Northwest and Canada (Detling, 1939). The two complexes intergrade geographically and morphologicallv. Sometimes northern association is segregated as D. brachycarpa (Richards) Schulz. The subspecies ha-ltctorum (Cockerell) Detl. includes both glandular and non-glandular plants; plants differ chromosome number. Cockerell (1901) gave specific rank to non-glandular one. The authors found glandular plant [Detling 2291, Boulder, Wyoming ; Detling 2300, Red Desert, Wyoming] to have 42 somatic chromosomes (fig. 7), non-glandular one [Detling 2360, Mesa Verde, Colorado] to have 28 somatic chromosomes (fig. 8). Likewise, subspecies glabra (Woot. & Standl.) Detl. [Detling 2387, Mojave Desert, California] was discovered to have a 2n-number of 28 (fig. 9). Subspecies Menziesii (DC.) Detl. was investigated by Manton (1932), who gave plant specific status and reported for it 28 somatlc chromosomes. The present authors counted chromosomes of three other subspecies: subspecies intermedia (Rydb.) Detl.