Delbert Wiens

Reproductive success, spontaneous embryo abortion, and genetic load in flowering plants

SummaryReproductive success is divided into two phases: preemergent (the number of viable seeds that enter the ambient environment) and postemergent (the percentage of progeny that survive to reproduce). We studied preemergent reproductive success (PERS) in flowering plants by measuring the fruit/flower (Fr/Fl) ratio and the seed/ovule (S/O) ratio in a number of species of outcrossing and inbreeding plants, where PERS=the product of (Fr/Fl) and (S/O). In order to determine the influence of the ambient environment (including resource availability) we studied pairs of outcrossing and inbreeding species occurring in the same habitat. Among outcrossing species PERS averaged about 22%, whereas in inbreeding species the average was approximately 90%. The progeny/zygote (P/Z) ratio was studied in hand-pollinated populations in Epilobium angustifolium (a strongly outcrossing species) from populations in Oregon and Utah, by direct observation of embryogenesis at twoday intervals throughout the course of seed development. The P/Z ratio in both populations averaged near 30%, and the developing embryos showed a surprising array of abnormalities that resulted in embryo death. During early development >95% of the ovules had normally developing globular embryos, but beginning with differentiation (cotyledon formation) about 70% of the original globular embryos aborted during the course of embryogenesis and seed development. The clustering of developmental lethals during peroids of major differentiation events parallels the animal model of development. We found little evidence that PERS was limited by the ambient environment (including resource availability), pollination, or factors associated with the inbreeding habit. Instead, PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability. The high incidence of developmental lethals in E. angustifolium and the resulting low P/Z ratio (ca. 30%) is attributed to genetic load (any lethal mutation or allelic combination) possibly working in combination with developmental selection (interovarian competition among genetically diverse embryos). Examples of maternally controlled, fixed patterns of ovule abortion with respect to position or number are discussed. However, we found no need to employ “female choice” as a hypothesis to explain our results for the extensive, seemingly random patterns of embryo abortion in E. angustifolium and other outcrossing species. A more parsimonious, mechanistic explanation based on genetic load-developmental selection is sufficient to account for the differential survivorship of embryos. Likewise, the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship.

Ovule survivorship, brood size, life history, breeding systems,and reproductive success in plants

The percentage of ovules developing into seeds (seed-ovule ratios, S/O ratios) is about 85% in annuals but only approximately 50% in perennials.In both annuals and perennials, these typical S/O ratios occur regardless of the kind of breeding system, although many annuals are normally self-pollinating whereas perennials are virtually all cross-pollinating. The mean number of seeds maturing within individual fruits is defined as brood size, and is correlated with different modes of dispersal and strategies of ovule packging. Annuals also have significantly higher brood sizes (21.7) than perennials (<9.9). Among perennials, woody plants have both lower S/O ratios (32.7%) and brood sizes (3.3) than herbaceous perennials (57.2%, 13.5). S/O ratios appear to be largely determined genetically, whereas resource limitations are perhaps more critical in terms of regulating flower production. Among perennials, increased exposure to predators and pathogens is suggested as the best explanation for theevolution of breeding systems favoring genetic recombination. The maintenance of genetic polymorphisms, however, inevitably increases the frequency of lethal and sub-lethal allelic combinations (and perhaps mutations?), that appear to be responsible for the lower S/O ratios in perennials.

Mimicry in Plants

Mimetic phenomena have played a central role in many evolutionary questions and today provide some of the best examples of natural selection. Since its discovery by Bates (1862) in Amazonian Heliconiid butterflies many hundreds of papers have been written on the subject and today it is recognized as a major feature of animal evolution, especially in insects.

Translocation heterozygosity and sex ratio in Viscum fischeri

SummaryMale plants of V. fischeri have 2n = 23 and constantly produce seven bivalents and a multivalent chain of nine chromosomes at meiosis. Regular assortment results in transmission of 11- and 12-chromosome genomes via the pollen. Female plants have the chromosome number 2n = 22 and are homozygous for the 11-chromosome genome. The multivalent chain in the males is a consequence of reciprocal translocations, one of which was Robertsonian and one of which involved the chromosome carrying the sex determination factors. There is a constant female-predominant sex ratio of approximately 1:2 in V. fischeri, possibly maintained by gamete selection; the genes involved may have been linked with the sex determination mechanism through the translocation system.

Permanent translocation heterozygosity in Viscum Album and V. Cruciatum : sex association, balanced lethals, sex ratios

SummaryPermanent translocation heterozygosity is confirmed in V. album in Europe, Japan, and California (where it is introduced), and reported for the first time in V. cruciatum from Israel. In V. album meiotic associations of are reported in male plants; in female plants meiotic configurations of 10II, 8II + ⊙IV, 7II + ⊙VI, 6II + ⊙VIII were discovered. In Japanese populations of V. album male plants have 5II+⊙X and 6II + ⊙VIII, but female plants have 6II + ⊙VIII and 7II + ⊙VI, indicating that in this population translocation heterozygosity may be maintained by a system of balanced lethals similar to those in Onagraceae. Other populations of V. album and V. cruciatum preserve permanent heterozygosity through a sex associated system previously described in other species. Female predominant sex ratios ranging from 0·49 to 0·24 occur in both V. album and V. cruciatum.

Permanent translocation heterozygosity and sex determination in East african mistletoes.

Viscum fischeri has 2n = 23 chromosomes in male plants. These fornm 7 bivalents and a translocation chain of 9 chromosomes during meiosis. Pollen with 11-and 12-chromosome genomes is thus produced. Female plants have 2n = 22 chromosomes and produce 11 bivalents during meiosis. Sex determination is technically a rare multiple X-multiple Y type, but more importantly it provides the mechanism whereby permanent translocation heterozygosity is maintained in the system. In a second species, Viscum engleri, male plants have 2n=28 chromosomes associating as 11 bivalents and a ring of 6 chomosomes at meiosis.

The classification of the generic segregates of Phrygilanthus (= Notanthera) of the Loranthaceae

The taxonomic composition ofPhrygilanthus has long been a source of confusion. Until recently the genus was considered to be distributed from Mexico, Central and South America, to Australia, New Guinea, and the Philippines. Analyses of floral morphology, inflorescence structure, life form, fruit type, and especially chromosome numbers indicate thatPhrygilanthus, as classically circumscribed, is a highly heterogeneous and artificial assemblage containing elements now referable to the following ten genera:Cecarria, Desmaria, Gaiadendron, Ligaria, Muellerina, Notanthera, Psittacanthus, Struthanthus, Tripodanthus, andTristerix. Phrygilanthus itself is nomenclaturally illegitimate and must be replaced by Notanthera. A synopsis of the genera segregated from thePhrygilanthus group is included. A key to the New World genera of Loranthaceae is also appended as well as a nomenclator indicating the present generic status of taxa at one time or another assigned toPhrygilanthus. One genus,Cecarria, is described as new; five new combinations are made:Cecarria obtusifolia (Merr.) Barlow,Psittacanthus palmeri (Wats.) Barlow & Wiens,Struthanthus panamensis (Rizzini) Barlow & Wiens,Tristerix aphyllus (DC.) Barlow & Wiens, andTristerix grandiflorus (Ruiz & Pavon) Barlow & Wiens.

Arceuthobium (Viscaceae) in Mexico and Guatemala: Additions and range extensions

Three new dwarf mistletoes are described:Arceuthobium globosum subsp.grandicaule (Mexico and Guatemala),A. aureum subsp.aureum (Guatemala) andA. aureum subsp.petersonii (Chiapas, Mexico).Arceuthobium guatemalense is recorded for the first time in Mexico. Significant range extensions are recorded forA. abietisreligiosae, A. divaricatum,A. gillii subsp.nigrum, andA. rubrum. New hosts are reported for several taxa. Nineteen members of the genus are presently known from Mexico, and three (possibly four) from Guatemala. Chromosome counts are reported for the first time for 3 taxa.

NEW TAXA AND NOMENCLATURAL CHANGES IN ARCEUTHOBIUM (VISCACEAE)

The following new taxa are recognized inArceuthobium: subgeneraArceuthobium andVaginata; sectionsVaginata, Campylopoda, andMinuta; seriesCampylopoda,Rubra, andStricta; speciesA.apachecum,A.californicum,A.guatemalense,A.hondurense, andA.pini; new formae spécialesA. abietinum f. sp.concoloris,A. abietinum f. sp.magnificae. New combinations:A.abietinum (Engelm.) Hawksworth & Wiens, andA.microcarpum (Engelm.) Hawksworth & Wiens.

Embryonic and host-associated skewed adult sex ratios in dwarf mistletoe

Embryonic sex ratios were determined for the first time in dioecious flowering plants by utilizing malate dehydrogenase (Mdh-3) as a genetic marker. In three populations of Arceuthobium tsugense ssp. tsugense (hemlock dwarf mistletoe) the embryonic sex ratio was 1:1, as it also was at first reproduction. Mixed-age, adult sex ratios of these and other populations from Oregon to south-eastern Alaska, however, were significantly female-biased (59 per cent) on western hemlock, its primary host. We suggest that these female-skewed, adult sex ratios are the result of increased longevity among females. On a secondary host, noble fir, the embryonic and the adult sex ratios were both 1:1. On still another secondary host, shore pine, the embryonic sex ratio is unknown, but the adult sex ratio was significantly male-biased (55.1 per cent). Host environment apparently influences adult sex ratios in hemlock dwarf mistletoe. However, in the European mistletoe Viscum album, a gametic system apparently controls the production of the adult female-predominant sex ratios (69.9 per cent) at fertilization (or perhaps by abortion during embryogenesis). In V. album, the sex ratio of progeny raised from seed is already similarly biased at first reproduction (67.3 per cent), and the sex ratios are not influenced by host shifts.