Combating plant diseases—the Darwin connection

Although Darwin knew of plant diseases, he did not study them as part of his analysis of natural selection. Effective plant disease control has only been developed after his death. This article explores the relevance of Darwin's ideas to three problem areas with respect to diseases caused by fungi: emergence of new diseases, loss of disease resistance bred into plants and development of fungicide resistance. Darwin's concept of change through natural or artificial selection relied on selection of many small changes, but subsequent genetic research has shown that change can also occur through large steps. Appearance of new diseases can involve gene duplication, transfer or recombination, but all evidence points to both host plant resistance and fungicide susceptibility being overcome through point mutations. Because the population size of diseases such as rusts and powdery and downy mildews is so large, all possible point mutations are likely to occur daily, even during moderate epidemics. Overcoming control measures therefore reflects the overall fitness of these mutants, and much resource effort is being directed towards assessment of their fitness, both in the presence and in the absence of selection. While recent developments in comparative genomics have caused some revision of Darwin's ideas, experience in managing plant disease control measures clearly demonstrates the relevance of concepts he introduced 150 years ago. It also reveals the remarkable speed and the practical impact of adaptation in wild microorganism populations to changes in their environment, and the difficulty of stopping or delaying such adaptation. Copyright © 2009 Society of Chemical Industry     

Darwin and the cirripedes: Insights and dreadful blunders

Charles Darwin's favorite animals were cirripedes (barnacles). Indeed, he worked intensively on cirripedes during the years he was maturing his thoughts regarding his theory, which eventually led to the publication of The Origin of Species. Here I present some of Darwin's achievements in the morphology, systematics and biology of these small marine invertebrates, and also, in light of our present knowledge, some mistakes he made.     

Charles Darwin, ichthyology and the species concept

This contribution presents the ichthyological writings of Charles Darwin (1809–1882), by periods, viz. ‘the years prior to the voyage of the Beagle’ (about 1825–1830); ‘the Beagle years’ (1831–1836); ‘from the return of the Beagle to the Foundation of Origin’ (1837–1844); and ‘the mature Darwin’ (1845–1882). Overall, this material covers 45 000 words penned by Darwin, but represents only 0.7% of his lifetime output of about 6 million words, indicating a limited interest in fish. However, this sample, briefly described here, but analysed in great detail in a forthcoming volume on Darwin's Fishes, allows drawing inferences on Darwin's working style that were missed in conventional biographies. On the other hand, it is suggested, based on a close reading of the 6^th (1876) edition of Origin, that Darwin was not particularly interested in the theoretical issues now associated with the species concept, nor indeed with other levels of the Linnean system.     

Species from Darwin onward

Controversy regarding the species problem has been going on for many decades and no consensus has ever been reached about what a “species” really is and how best to define the concept. De Queiroz (1998) introduced a distinction between two aspects of this problem: on the one hand, the definition proper, and on the other, the criteria allowing biologists to recognize species in practice. This distinction is a first step on the way toward a solution of the problem. In the present paper, we show that de Queiroz's distinction is made possible by the radical theoretical change introduced by Darwin. We emphasize that the species problem did not appear in the 20^th century, but long before, and that Darwin addresses it indirectly in the Origin of Species. It might seem paradoxical to refer to Darwin's views about species, because they are usually considered as unclear. However, we propose that an analysis of these views in the context of Darwin's own theory of evolution might reveal how a definition of the concept of species is made possible by being anchored to the very theory of evolution. To this aim, we present a plausible reconstruction of Darwin's implicit conception of species and show how this conception fits with the debates on species that took place in the 18^th and 19^th centuries. We then turn to today's biology and show what changes Darwin's implicit conception of species has brought about relative to the species concept and species delimitation.     

Of trees,geese and cirripedes: Man's quest for understanding

At least zoologists know that barnacles are arthropods rather than mollusks. However, this knowledge is surprisingly new, for it was as recent as 1830 before J. Vaughan Thompson showed, through a careful study of barnacle larvae, that they were crustaceans. In the 1850s, Charles Darwin unraveled much of the taxonomy of barnacles, and, significantly, his observations and classification of them follow the structure that was to be published later as his evolutionary theory. Irrespective of these works, knowledge of the systematic placement of barnacles remains surprisingly poor in the wider population today, with most non‐biologists viewing barnacles as shallow‐water fouling organisms related to oysters and limpets. The present paper reviews the way humans have perceived barnacles for at least a millennium; it evaluates why they were thought to have grown from trees and to have been part of the life cycle of birds; it concludes by contemplating the manner in which we perceive our environment and by doing so try to make sense of our world.     

The origin of Darwin hybrid tulips analyzed by flow cytometry, karyotype analyses and genomic in situ hybridization

Chromosome morphology was studied in diploid cultivars of Tulipa fosteriana and T. gesneriana (2n = 2x = 24) and triploid Darwin hybrids (2n = 3x = 36) developed from interspecific crosses of T. gesneriana and T. fosteriana. Chromosomes were arranged in the karyotype according to decreasing total length. Based on our karyotypic analysis, we propose that median chromosomes may serve as markers for diploid genotypes. Discriminant analysis with respect to total chromosome length and short arm length showed a significant difference between the size of the larger median chromosomes of T. gesneriana and T. fosteriana Comparison of median chromosome length in Darwin hybrid tulips showed that two larger chromosomes and one smaller chromosome were derived from T. gesneriana and T. fosteriana, respectively. This finding was clearly and unambiguously confirmed by simultaneous hybridization of differentially labeled genomic probes of T. fosteriana and T. gesneriana to metaphase chromosomes of the triploid cultivar ‘Yellow Dover’, thereby enabling us to distinguish between the 24 chromosomes derived from T. gesneriana and 12 chromosomes derived from T. fosteriana. Thus, genomic in situ hybridization and median chromosome analyses can be useful to identify the genome constitution of triploid Darwin hybrid tulips. In addition, their hybridity was readily verified by flow cytometry using vegetative tissue of Darwin hybrid tulips. Our results clarify the process of Tulipa cultivar formation and will be useful for interspecific hybridization breeding.The first and second author have contributed equally to this paper     

Aspects of unconscious selection and the evolution of domesticated plants

Summary Unconscious selection may be defined as non-intentional human selection. The term was introduced by Darwin and its modern concept was developed by C.D. Darlington. Unconscious selection, or automatic selection as it is sometimes called, could have been responsible for most of the differences that distinguish domesticated seed crops from their wild progenitors, including loss of natural dispersal mechanisms, even and rapid seed germination, larger propagules, simultaneous ripening, and loss of mechanical protection as well as changes in the breeding system. Some differences, such as those in seed or fruit colors, may have developed from conscious selection at an early time. For unconscious selection to operate in the development of domesticated plants there would have to have been a deliberate planting of seeds by people.     

Developing native fish species for aquaculture: the interacting demands of biodiversity, sustainable aquaculture and livelihoods

Aquaculture continues to be the fastest growing animal production industry and this rate of expansion must continue if aquaculture is to satisfy global demand for fish products in the face of dwindling capture fisheries. The relationship between aquaculture and biodiversity is complex, with examples of positive and negative impacts having been reported. To enable this expansion while avoiding negative impacts from introductions of exotic species, the investigation of indigenous species becomes important and worthwhile.This paper establishes the background to development of new species for culture and describes the example of the Mexican silverside Menidia estor, which has for centuries been the principal species in an artesanal fishery in Lake Pátzcuaro, Mexico. The species is geographically isolated and is unique but is now endangered because of a range of factors including overfishing, environmental degradation and introduction of exotic species. Considerable advances have been made recently in developing a closed reproductive cycle, understanding feeding and small‐scale on‐growing technology for the species. Based on this, a Darwin Initiative programme was developed focused on technology transfer to implement small‐scale pilot on‐growing thus helping to conserve the species and to improve livelihoods. This has allowed successful pilot scale development of aquaculture for the species while at the same time addressing the objectives of the international Convention on Biodiversity.