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Systematics |
Biology Department, Santa Clara University, Santa Clara, California 95053 USA
Received for publication March 12, 2002. Accepted for publication June 11, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: Asteraceae • Eriophyllum • experimental hybridizations • Helenieae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). The biosystematic study reported here has its roots in Constance's (1937)
alpha taxonomic study of Eriophyllum and Carlquist's (1956)
anatomical and cytotaxonomic study of Eriophyllum, Pseudobahia, Syntrichopappus, and Monolopia. The results of the study described here complement the molecular studies of Baldwin and others (e.g., Baldwin and Wessa, 2000
). I dedicate this paper to the memory of Lincoln Constance (1909–2001) and to Sherwin Carlquist.
The annual and perennial species of Eriophyllum range from British Columbia to Mexico along the Pacific Coast and east to Utah and Wyoming, in communities as different as seashore, desert, and subalpine forest. Some species differ markedly from one another, while others are difficult to distinguish at arm's length; even delimiting related genera can be difficult. Jepson (1925)
, for example, treated Pseudobahia heermannii as Eriophyllum heermannii. The presence of relatively few reliable characters contributed to about 157 taxonomic designations for the published species of Eriophyllum between 1890 and 1937 (Constance, 1937
). Constance (1937)
pruned this taxonomic thicket to 11 species and 12 varieties. Mooring and Johnson (in Hickman, 1993
) recognized six perennial and eight annual species. Molecular systematics has begun to change that treatment (Baldwin, 1999
; Baldwin and Wessa, 2000
).
Six perennial taxa (Eriophyllum confertiflorum var. tanacetiflorum, E. jepsonii, E. latilobum, E. nevinii, and E. lanatum vars. hallii and obovatum) and three annual species (E. congdonii, E. mohavense, and E. nubigenum) are rare and endangered (Tibor, 2001
).
The annuals are a disparate group. Individuals may be 1 (E. mohavense) to 30 cm tall (E. ambiguum, E. congdonii), decumbent-ascending (E. multicaule) to strictly ascending (E. nubigenum), with discoid (E. mohavense, E. pringlei) or radiate heads. They range from central California to northern Mexico and east to Utah in mesic to xeric plant communities (Table 1).
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), remains unknown. Numbers for the other annual species have been published (Carlquist, 1956
; Strother, 1972
, 1976
; Keil and Pinkava, 1976
; Johnson, 1978
; Mooring, 1986
): 2n = 14 for E. ambiguum, E. congdonii, E. nubigenum, E. multicaule, and E. pringlei, 2n = 10 for E. wallacei, and 2n = 8 for E. lanosum. Chromosome number is associated with geographic distribution. Constance (1937
, p. 73) portrayed the closely related E. congdonii and E. nubigenum as ancestral to the other annual species. Eriophyllum congdonii and E. nubigenum, both 2n = 14, appear to be restricted to foothill woodland and coniferous forest communities in and near Yosemite National Park, California. Eriophyllum ambiguum, E. multicaule, and E. pringlei, also 2n = 14, occur to the south in more arid communities in California, Nevada, and Arizona; E. wallacei (2n = 10) and E. lanosum (2n = 8) are found in warm desert communities from central California to northern Mexico (Table 1).
Chromosomes in excess of the normal complement, both full sized and "B," have been reported in E. pringlei and E. wallacei (Strother, 1972
, 1976
; Keil and Pinkava, 1976
; Johnson, 1978
). About 80 populations have been sampled, mostly E. wallacei (
33 populations) and E. lanosum (13 populations). Additional sampling might discover extra chromosomes in the other species, notably E. multicaule, represented by only four populations.
The present study arose from two earlier artificial hybridization studies. One of these (Mooring, 2001
) found barriers to interbreeding among diploid populations of the Eriophyllum lanatum species complex. However, barriers to interbreeding between E. congdonii and E. nubigenum were not found in a second study (Mooring, 1991
). The contrast of barriers in a perennial species and absence of such barriers in two annual species led to the present study of crossing relationships among the other annual taxa.
In this paper, I present results of experimental hybridizations among the annuals E. ambiguum var. ambiguum, E. ambiguum var. paleaceum, E. congdonii, E. multicaule, E. nubigenum, E. pringlei, E. lanosum, and E. wallacei, as well as between E. ambiguum and E. congdonii and the perennial species E. lanatum.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Treatments
Fruits were germinated in vermiculite or in vermiculite-soil mixtures. Seedlings were potted in "UC Mix" soil in an unheated, pollinator-free Santa Clara University greenhouse. Generally, cultivation was accomplished without problems, except for whitefly infestations in the greenhouse. Some plants were also transplanted to garden beds.
Fruit quality estimates
Fruit color ranges from dark brown through straw. Dark brown fruits may or may not have an embryo, lighter colored ones generally do not, and the lightest colored ones always lack an embryo. I classified fruit quality as "good" (dark and stiff), "fair" (moderately dark and stiff), "poor" (pale and flexible), and "NG" (obviously without an embryo). Varying proportions of "good" and "fair" fruits failed to germinate. Germination data, therefore, must be interpreted with that caveat in mind.
Pollen viability estimates
Pollen samples from species other than E. nubigenum were obtained by gently tapping a head with mature anthers over a slide, then adding a drop of cotton blue-lactophenol. Eriophyllum nubigenum, unlike its congeners, is self-pollinating and produces and releases many fewer pollen grains per head (Table 1); its anthers were dissected in cotton blue-lactophenol. Pollen grains were examined at 40x or 400x magnification after being stained overnight in cotton blue-lactophenol. Over 95% of the estimates of pollen stainability rest on 300+ grains per sample, with each plant being sampled on two different days. The other estimates are based on 17–270 pollen grains.
Meiotic examinations
Young heads of garden or greenhouse plants were fixed in 1 : 3 acetic ethanol. Quickly placing collections in an ice-filled cooler seemed to improve fixation (Anderson, 1966
). Most cytological studies were of diakinesis or M1 stages of microsporocytes squashed in acetocarmine and examined with a phase-contrast microscope at 1000x magnification. Beeks' (1955)
technique provided clearer preparations. Voucher specimens have been deposited in the Santa Clara University herbarium (SACL), and duplicates of most of them will be distributed elsewhere.
Compatibility
Compatibility was assessed in the greenhouse by three methods. (1) Test plants were isolated (separated by at least 15 cm). (2) Isolated plants were caged in a fine-meshed cylinder. (3) Isolated or caged plants were artificially self-pollinated 1–2 times per day over 3–5 d. Fruits from test plants were then examined and rated for fruit quality and, where possible, planted and tested for germination.
Artificial hybridizations
Heads of isolated plants in the greenhouse were rubbed together 1–2 times per day over 3–5 d. The percentage of stainable pollen in the plants used as parents generally exceeded 90% (Table 1).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Eriophyllum nubigenum and E. congdonii are self-compatible (Table 2), the former more so than the latter, confirming an earlier report (Mooring, 1991
). In nature, E. nubigenum resembles a reduced E. congdonii, being about half its stature and with inconspicuous ligules. In greenhouse and garden, E. nubigenum habitually self-pollinates, produces and releases comparatively small amounts of pollen, and seems to be relatively unattractive to pollinators. Eriophyllum congdonii can self-pollinate, produces and releases large amounts of pollen (Table 1), and attracts pollinators, but it often produces more fruits when cross-pollination occurs. Selfing did not decrease vigor and fertility in either species. In E. congdonii, the pollen stainability of eight vigorous second generation plants was 85–99% ( = 95%).
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Hybrid fertility, as estimated by pollen stainability, averaged 83% in the progeny of self-compatible E. congdonii x E. nubigenum, but only 1–16% in the hybrids resulting from crosses involving self-incompatible species. Diakinesis and M1 configurations in interspecific hybrids ranged from 14 I to 7 II (Table 3). Analysis at pachytene might have revealed other differences. Jackson (1984)
has commented on the loss of possible information that could result from studies restricted to post-pachytene stages. Root-tip chromosomes were not studied. The cause or causes of reduced pairing in these hybrids is unknown. Chromosomal rearrangements are a possibility, as suggested by the presence of three base chromosome numbers in the annual species, the frequency of extra chromosomes in two species, and variations in base chromosome numbers in the related genera Pseudobahia, Syntrichopappus, and Monolopia (Johnson, 1978
). Genetic control of pairing (Jackson, 1982
) is possible and is the usual cause of "lack of or reduced pairing in hybrids" (R. Jackson, Texas Tech University, personal communication).
The combinations producing hybrids are discussed below, grouped alphabetically under self-compatibility and self-incompatibility.
Hybrids between self-compatible taxa
Eriophyllum congdonii x E. nubigenum
Both species are self-compatible (Table 2); E. congdonii is outcrossing and a copious pollen producer, and E. nubigenum is self-pollinating and produces and releases comparatively little pollen (Table 1). Where E. congdonii was the pollen parent, hybrids comprised 47 of the 78 progeny examined; hybrid pollen stainability was 73–99% ( = 83%), and 7 bivalents or, infrequently, 6 II + 2 I were formed at meiosis (Table 3). Where E. nubigenum was the pollen parent, indisputable hybrids were not detected among the 37 progeny examined. The apparent lack of hybrids here contrasts with my earlier findings (Mooring, 1991
), when I reported an array of vigorous and fertile intermediate individuals. The interfertility between these species contrasts sharply with the strong barriers to interbreeding among the other annual species (Table 3).
Taxonomic treatments have differed. Jepson (1925)
treated E. congdonii without mentioning E. nubigenum. Constance (1937)
placed E. congdonii as a variety of E. nubigenum. Abrams and Ferris (1960)
ranked each as species. Constance (1937
, p. 115) commented that the taxa "present an interesting problem in determination of specific delimitation." He attributed their morphological differences to higher vs. lower elevation environments. Mooring (1991)
recommended treating each as a species, despite their interfertility, citing three scientific and one conservation consideration: (1) Morphological distinctions were maintained in greenhouse cultures; therefore the differences are genetic, not environmental modifications as Constance (1937)
believed. (2) Eriophyllum nubigenum is strongly self-pollinating and produces and releases comparatively little pollen; interbreeding is unlikely in nature. (3) A nearest-neighbor distance of 11 km, pollinator preferences, and differences in flowering time make interbreeding under natural conditions unlikely. (4) Both taxa are rare (Tibor, 2001
); specific status was more likely to result in better protection. Then, espousing specific status was reasonable because no information about intersterility among the other annual species had been reported. Now, strong barriers to intercrossing have been found among the other species; subspecific status would be reasonable.
Hybrids between self-incompatible taxa
Eriophyllum ambiguum var. ambiguum x E. ambiguum var. paleaceum
Johnson (in Hickman, 1993
) used disc corolla pubescence to distinguish these varieties. Herbarium specimens of the parental plants displayed the difference; those of the progeny did not. Pollen stainability percentage in the 14 progeny of var. ambiguum x var. paleaceum was 0% (three plants) to 97% ( = 54 ± 40%). Meiosis was not studied (Table 3). The reciprocal cross produced only ten apparently viable fruits, of which two germinated but died. Considerable variation exists among E. ambiguum populations; Gray, Brandegee, and Rydberg recognized some variants as species (Constance, 1937
). More hybridizations might have detected barriers to interbreeding.
Eriophyllum ambiguum var. ambiguum x E. pringlei
Eriophyllum ambiguum has large, radiate heads; E. pringlei has small, discoid heads and is usually less than one-quarter as tall as the former (Table 1). These crosses were the only ones that produced hybrids (six of the eight progeny), no matter which species served as the seed parent. Pollen stainability was nearly the same in the reciprocal crosses ( = 13% vs. 16%), but percentage germination (4% vs. 89%) and the frequency of meiotic configurations differed with the direction of the cross. With E. ambiguum var. ambiguum as the seed parent, the usual meiotic configuration was seven loose pairs. The univalents were positioned in pairs and close to each other, but not visibly connected. I have previously (Table 3, Mooring, 2001
) referred to this arrangement of chromosomes in Eriophyllum lanatum artificial hybrids as "loose pairs." However, reviewer R. C. Jackson (Texas Tech University, personal communication) observes that no bivalent exists unless the chromosomes are held together and recommends referring to "close proximity of two suspected univalent homologues or homoeologues." For purposes of brevity, I shall continue to use the term "loose pairs" without implying bivalent formation. Other configurations were 6 II + 2 I, 4 II + 6 I, 3 II + 8 I, and 2 II + 10 I. With E. pringlei as the seed parent, seven "loose pairs" or, less frequently, 5 II + 4 I were formed (Table 3). The "loose pair" configuration consistently appears; it appears to be real, rather than an artifact produced by excess pressure in squash procedures.
Eriophyllum ambiguum var. paleaceum x E. multicaule
The distance from ligule tip to ligule tip of the desert-dwelling E. ambiguum is up to five times that of the coastal E. multicaule, and E. multicaule produces considerably less pollen (Table 1) and is slightly self-compatible (Table 2). Only selfs were produced when E. multicaule was the pollen parent. The reciprocal combination consisted of three crosses that produced nine progeny. Two of the three crosses produced a hybrid. The heads never opened in one, and in the other, only six of 17 pollen grains in the sample stained (Table 3).
Eriophyllum ambiguum var. paleaceum x E. pringlei
The conspicuous morphological differences between these species have been noted above. No hybrids were generated when E. pringlei was the seed parent. In the reciprocal cross, the only hybrid in 39 progeny did not develop microsporocytes. These results contrast with those in the E. ambiguum var. ambiguum x E. pringlei cross described above. There, hybrids were produced no matter which species served as the seed parent, and six of eight progeny were hybrids, including five of six progeny when var. ambiguum was the seed parent (Table 3).
Eriophyllum multicaule x E. pringlei
Both species have small heads. The former is slightly self-compatible, the latter is self-incompatible. Radiate E. multicaule came from a coastal site; eradiate E. pringlei came from a desert population (Tables 1, 2). With E. multicaule as seed parent, four crosses involving eight parents produced 148 fair-to-good fruits. In the single cross that generated hybrids, ten of the 90 fair-to-good fruits germinated. Two of the ten progeny were hybrids, with pollen stainabilities of 1% and 7%. The single plant whose meiosis was examined formed 2 II + 10 I, 3 II + 8 I, and 5 II + 4 I (Table 3). No germination occurred in the 11 reciprocal crosses.
Eriophyllum lanosum (n = 4) x E. wallacei (n = 5)
This heteroploid combination may have yielded hybrids. No germination occurred when E. lanosum was the seed parent. In contrast, a total of 124 apparently viable fruits came from the two crosses when E. wallacei was the seed parent; all the 25 progeny appeared to be selfs (Table 3). Three of the selfs, however, all from the same parents, had pollen stainabilities of 17%, 32%, and 35%; significantly less than 53%, the lowest percentage of stainable pollen in the greenhouse plants from which the parents came. It is possible that these three are hybrids closely resembling the seed parent rather than being approximately intermediate. In other species, hybrids may resemble one of the parents (Ornduff, 1969
; Raven, 1976
; Stace, 1980
, p. 140). Constance (1937
, p. 118) regarded these species as "intimately" related, perhaps derived from the same source as E. ambiguum.
Hybrids between self-compatible and self-incompatible taxa
Eriophyllum ambiguum var. paleaceum x E. congdonii
In the greenhouse, these widely separated species (Table 1) closely resemble each other at arm's length; they differ in pappus scale length and anther tip pubescence. Using the self-compatible E. congdonii as seed parent produced 455 fruits, of which 212 germinated; nine of the 155 resulting seedlings planted were hybrids. Seven hybrids produced no pollen; pollen stainability in plant 41 and plant 45 averaged, respectively, 12% and 32%. The most frequent meiotic configuration in plant 41 was 2 II + 10 I, others were 14 I, 1 II + 12 I, 3 II + 8 I, and, probably, 5 II + 4 I. In plant 45, the most frequent configuration was 1 II + 12 I; others were 14 I or 2 II + 10 I. The reciprocal cross generated only one apparently viable fruit; it did not germinate (Table 3).
Eriophyllum ambiguum var. paleaceum x E. nubigenum
Eriophyllum ambiguum is large-headed, open-pollinated, and produces much pollen; E. nubigenum is small-headed, self-pollinated, and produces and releases comparatively little pollen (Table 1). Where E. nubigenum was the pollen parent, only four "fair" looking fruits were produced and the resulting two seedlings were selfs. Where E. nubigenum was the seed parent, 48 of the 175 apparently viable fruits germinated, and 1 of the 36 seedlings planted was a hybrid. Pollen stainability was 5%, and meiotic configurations of 3 II + 8 I or 4 II + 6 I were most frequent, with an occasional 2 II + 10 I (Table 3). Some telophase1 cells had chromosomes in the middle as well as at the poles.
Eriophyllum congdonii x E. multicaule
With the self-compatible and copious pollen producer E. congdonii as seed parent, five crosses yielded a total of 133 apparently viable fruits, of which 18 germinated. The 12 survivors were selfs. With the slightly self-compatible (Table 2) and moderate pollen producer E. multicaule as seed parent, two of the four crosses produced a total of 36 fair-to-good fruits; one germinated, but the seedling died before planting. The third cross produced 29 fair-to-good fruits; the four progeny were selfs. In the fourth cross, 43 of the 117 fair-to-good fruits germinated, and 11 progeny were hybrids. Ten of the hybrids furnished data on pollen viability; four had no pollen, four had no stainable pollen, and the other two averaged 4.2% and 11.3% stainable pollen. The single plant whose meiosis was examined formed from 2 II + 10 I to 6 II + 2 I, and some cells were scored as having 42 chromosomes, apparently by mitotic divisions (Table 3). Carr, Baldwin, and Kyhos (1996)
reported mitotic replacing meiotic divisions in a Daubautia x Raillardiopsis hybrid.
Eriophyllum congdonii (n = 7) x E. lanatum var. grandiflorum (n = 8)
Six crosses, including one reciprocal, were made in this heteroploid combination between a self-compatible annual and a self-incompatible perennial. The crosses involved two populations of each species and eight parents. Eriophyllum congdonii was the seed parent in two crosses. In one progeny, a probable self resembled E. congdonii, but the two pollen samples totaled only 119 grains, and the grains were misshapen (Table 3). Meiosis was not studied. In the Trumbull Peak x Pilot Peak cross, however, 16 of the 20 fair-to-good fruits germinated, but six plants died before flowering. The ten survivors, all hybrids, resembled the pollen parent (E. lanatum var. grandiflorum) more than they did the seed parent. Three plants produced no pollen; five had hundreds of unstained grains; two had 100% stained grains, 18 from one plant and 23 from the other. The single plant whose meiosis could be analyzed formed 15 I at metaphase and produced no pollen (Table 3). Eriophyllum lanatum var. grandiflorum was the seed parent in the other four crosses. Pilot Peak x Trumbull Peak produced 257 fair-to-good fruits, but none germinated. The other three crosses yielded a total of six seedlings. The three plants that survived vandalism were selfs, with pollen stainability 58–98%.
Barriers to interbreeding
Assuming that the individuals used in the experimental hybridizations were approximately typical of the populations, and that the populations were approximately typical of the species, strong barriers to intercrossing exist among the annual species, except for the interfertility exhibited in E. congdonii x E. nubigenum crosses. The barriers to interbreeding (Table 3, Fig. 1) are far stronger than the strong-to-weak barriers among the diploid "varieties" of the E. lanatum complex and among them and E. confertiflorum (Mooring, 2001
).
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).
Relationships to other genera
The haploid chromosome numbers of n = 4, 5, and 7 in the annual and n = 8 in the perennial Eriophyllum species are close to, or the same as the haploid numbers in the related genera Pseudobahia (n = 3, 4, 5, 8) and Syntrichopappus (n = 6, 7). Grant (1981)
suggested descending dysploidy to explain the 8–7-()-4–3 sequence for haploid numbers in Eriophyllum and Pseudobahia, the () representing missing numbers.
Relationship of the Eriophyllum annuals to the perennial species
Eriophyllum has six haploid chromosome numbers, n = 7, 5, and 4 in the annual species, and three haploid numbers in the perennials, n = 8 (E. lanatum, E. confertiflorum, E. latilobum, E. jepsonii), n = 15 (E. staechadifolium), and n = 19 (E. nevinii). Eriophyllum lanatum and E. confertiflorum are polyploid complexes (Mooring, 1994
, 2001
). Eriophyllum latilobum and E. jepsonii are polyploids probably derived by hybridization from the first two (Constance, 1937
; Munz, 1959
), and E. staechadifolium and E. nevinii probably are paleopolyploids.
The pattern of lower chromosome numbers in the annual species and higher ones in the perennials agrees with Stebbin's (1950
, pp. 167–170) discussion of chromosomal mechanisms and genetic systems. Lower chromosome numbers tend to go with the annual mode, higher ones with the perennial habit, thus balancing fitness and evolutionary flexibility. The pollination mode in annual eriophyllums, however, seems to differ from Stebbin's (1950
, p. 168) generalization that a "large proportion of annual species are predominately self-pollinated." Instead, E. ambiguum, E. multicaule, E. pringlei, and E. wallacei are self-incompatible (Table 2), and E. congdonii, though self-compatible (Table 2), sets a higher proportion of viable fruits when cross- rather than self-pollinated. As regards habitats, the perennials and the annuals E. congdonii and E. nubigenum (both n = 7) live in more mesic environments than the other annuals with n = 7 and E. wallacei (n = 5) and E. lanosum (n = 4).
Phylogeny
Mooring (1997)
used chromosome number, geographic distribution, cytogeography, habitat considerations, and the results of artificial hybridizations to hypothesize a descending dysploidy phylogeny 19-()-15-()-8–7-()-5–4 from the perennial to the annual eriophyllums (Fig. 2). Originating from an E. nevinii-like stock (n = 19), the hypothetical phylogeny passes through taxa ancestral to E. staechadifolium (n = 15), E. confertiflorum (n = 8), E. lanatum (n = 8), E. congdonii (n = 7), and E. ambiguum (n = 7). The lineage then diverges to separate ones leading to E. multicaule (n = 7) and E. pringlei (n = 7), on the one hand, and to E. wallacei (n = 5) and E. lanosum (n = 4), on the other. Efforts to obtain artificial hybrids between E. staechadifolium (n = 15) and tetraploid stocks (n = x = 16) of E. lanatum or E. confertiflorum populations have been unsuccessful but will continue, as will crosses involving the annuals E. congdonii and E. ambiguum (n = 7) and diploid (n = 8) populations of the perennial species E. confertiflorum and E. lanatum.
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, relying on morphology, rDNA analyses, and chromosome number, erected the genus Constancea on E. nevinii. In Baldwin's Fig. 1 (Baldwin and Wessa, 2000
, p. 1906) Constancea (Eriophyllum) nevinii is shown nested between the Baeriinae (including Eriophyllum, Monolopia, Pseudobahia, and Syntrichopappus) and n = 19 taxa (e.g., Arnica, Hulsea).
Clearly, taxonomic realignments (Baldwin, 1999
; Baldwin and Wessa, 2000
, p. 1906) require more molecular and biosystematic studies in Eriophyllum and related genera.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Anderson L. 1966 Cytotaxonomic studies in Chrysothamnus (Astereae, Compositae). American Journal of Botany 53: 204-212[CrossRef][ISI]
Baldwin B. 1999 CONSTANCEA, a new genus for Eriophyllum nevinii (Compositae-Heliantheae s. lat). Madroño 46: 159-160
Baldwin B. B. Wessa 2000 Origin and relationships of the tarweed-silversword lineage (Compositae-Madiinae). American Journal of Botany 87: 1890-1908
Beeks R. 1955 Improvements in the squash technique for plant chromosomes. Aliso 3: 131-134
Carlquist S. 1956 On the generic limits of Eriophyllum (Compositae) and related genera. Madroño 13: 226-239
Carr G. B. Baldwin D. Kyhos 1996 Cytogenetic implications of artificial hybrids between the Hawaiian silversword alliance and North American tarweeds (Asteraceae: Heliantheae—Madiinae). American Journal of Botany 83: 653-660[CrossRef][ISI]
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Jackson R. 1982 Polyploidy and diploidy: new perspectives on chromosome pairing and its evolutionary implications. American Journal of Botany 69: 1512-1523[CrossRef][ISI]
Jackson R. 1984 Chromosome pairing in species hybrids. In W. F. Grant [ed.], Plant systematics, 67–86. Academic Press, New York, New York, USA
Jepson W. 1925 A manual of the flowering plants of California. Associated Students Store, University of California, Berkeley, California, USA
Johnson D. 1978 Systematics of Eriophyllinae (Compositae). Ph.D. dissertation, University of California, Berkeley, California, USA
Johnson D. 1993 Eriophyllum. In J. Hickman [ed.], The Jepson manual: higher plants of California, 261–267. University of California Press, Berkeley, California, USA
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Mooring J. 1986 Chromosome counts in Eriophyllum (Compositae: Helenieae). Madroño 33: 186-188
Mooring J. 1991 A biosystematic study of Eriophyllum congdonii and E. nubigenum (Compositae, Helenieae). Madroño 38: 213-226
Mooring J. 1994 A cytogenetic study of Eriophyllum confertiflorum (Compositae, Helenieae). American Journal of Botany 81: 919-926[CrossRef][ISI]
Mooring J. 1997 A new base chromosome number and phylogeny for Eriophyllum (Asteraceae, Helenieae). Madroño 44: 364-373
Mooring J. 2001 Barriers to interbreeding in the Eriophyllum lanatum (Asteraceae, Helenieae) species complex. American Journal of Botany 88: 285-312
Mooring J. D. Johnson 1993 In J. Hickman [ed.], The Jepson manual: higher plants of California, 261–267. University of California Press, Berkeley, California, USA
Munz P. 1959 A California flora. University of California Press, Berkeley, California, USA
Ornduff R. 1969 The systematics of populations in plants. In Systematic biology, Proceedings of an International Conference, 104–128. National Academy of Sciences, Washington, D.C., USA
Raven P. 1976 Systematics and plant population biology. Systematic Botany 1: 284-316[CrossRef]
Stace C. 1980 Plant taxonomy and biosystematics. Edward Arnold, London, UK
Stebbins G. 1950 Variation and evolution in plants. Columbia University Press, New York, New York, USA
Strother J. 1972 Chromosome studies in western North American Compositae. American Journal of Botany 59: 242-247[CrossRef][ISI]
Strother J. 1976 Chromosome studies in Compositae. American Journal of Botany 63: 247-250[CrossRef][ISI]
Tibor D. [ed.] 2001 Inventory of rare and endangered plants of California, 6th ed. California Native Plant Society, Sacramento, California, USA
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