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O Cuellar

Publications and source records attributed to O Cuellar.

10 recordsLinked to original sources
PubMed1977

Animal parthenogenesis.

The available evidence on the ecological factors favoring the existence and origin of natural parthenogenesis is evaluated. Analysis of the geographical distributions of the well-known cases of animal parthenogenesis in nature reveals (i) that most of these species exist in natural disclimax communities and (ii) that within these communities they exist in isolation from closely related congeneric species. Parthenogenesis can only evolve in areas devoid of the generating bisexual species, because such species would prevent newly formed unisexuals from establishing clones due either to hybridization or competition. Furthermore, the two unique features allowing parthenogenetic species to invade and occupy open habitats faster than bisexuals are (i) a double intrinsic rate of increase and (ii) the ability of one individual to establish a new colony.

Animals↗
PubMed1977

Analysis of histoincompatibility in a natural population of the bisexual whiptail lizard Cnemidophorus tigris.

Histoincompatibility was analyzed to describe as precisely as possible the sequence of gross morphological events taking place during allograft rejection among closely related conspecifics from a natural population of the bisexual lizard Cnemidophorus tigris. Two types of rejection were noted, depending on whether the grafts were sloughed or not. Abrupt rejection was typically characterized by hemorrhaging of the graft site, scale dissolution, and eventual sloughing of the graft. This process occurred in a graded sequence. Acute rejection occurred from 15 to 20 days, subacute from 30 to 45, and subchronic from 60 to 90. In the gradual form of rejection, the allografts were gradually replaced by host tissue during a period ranging from approximately 100 to 350 days post-transplantation. Such a gradation observed in both the abrupt and gradual categories suggests that large numbers of genes and/or alleles are responsible for the antigenic properties of skin in these lizards.

Animals↗
PubMed1976

Cytology of meiosis in the triploid gyngenetic salamander Ambystoma tremblayi.

Female meiosis was analyzed in the triploid gynogenetic salamander Ambystoma tremblayi to determine the mechanism by which a stable chromosome number is maintained in this unisexual species. Gross details of the reproductive cycle and the cytology of meiosis were analyzed in 20 specimens and 320 oocytes involving all stages from early diplotene to the beginning of anaphase II. Ovulation apparently continues progressively involving a few oocytes at a time. Oocytes form the ovary contained chromosomes in diplotene, and diakinesis. The first metaphase was not observed since this stage occurs swiftly either immediately prior to or during ovulation. Oocytes in the most anterior region of the oviduct were in metaphase II, and those in the most posterior region were undergoing the beginning of anaphase II. Telophase II was not observed. Chromosome numbers obtained at all stages of prophase gave counts of approximately 42 bivalents, equivalent to the triploid somatic number known for this species. Similar numbers of dyads were obtained from metaphase II plates. This analysis supports earlier evidence suggesting that the triploid number of chromosomes in oocytes of A. tremblayi is doubled prior to meiosis, and the somatic number is restored by two normal meiotic divisions.

Ambystoma↗
PubMed1976

Intraclonal histocompatibility in a parthenogenetic lizard: evidence of genetic homogeneity.

A total of 175 skin grafts were transplanted among 20 individuals belonging to two separate populations of the parthenogenetic lizard Cnemidophorus uniparens. Of these, 98.8 percent were permanently accepted, which indicates that all individuals of each population may be genetically identical. These results further suggest that large populations or the entire species may consist of one clone derived from a single individual.

Animals↗
PubMed1976

Natural hybridization between parthenogenetic and bisexual lizards: detection of uniparental source of skin grafting.

Skin graft evidence is used to directly identify the unisexual parent of natural hybrids produced between the bisexual species Cnemidophorus inornatus and the unisexual Cnemidophorus neomexicanus. Reciprocal grafts transplanted among there suspected hybrids between Cnemidophorus inornatus and Cnemidophorus neomexicanus are rejected as well as grafts from the hybrids to C. neomexicanus, indicating that the hybrids are each genetically different from each other and that C. neomexicanus has the ability to reject foreign grafts. Allografts among C. neomexicanus and xenografts from C. neomexicanus to the hybrids are not rejected, indicating that C. neomexicanus is genetically homogeneous and that the hybrids possess genomes genetically identical to C. neomexicanus. Electrophoretic analysis of the hybrids and their probable parents supports the evidence obtained from skin grafting that C. neomexicanus is one of the parents, and suggests that C. inornatus, rather than C. tigris, is the bisexual parent. Chromosome spreads obtained from the hybrids reveal a triploid number of 69, a number consistent with their presumed origin from unreduced diploid ova (2N = 46) from C. neomexicanus and haploid serum (N = 23) from C. inornatus. Preliminary evidence is presented of the first hybrids known between Cnemidophorus inornatus and Cnemidophorus uniparens. Histological examination of the tests suggests that these hybrids are sterile. Included is a list summarizing all known hybrids between bisexual and unisexual species of the genus Cnemidophorus.

Animals↗
PubMedDate not supplied

[Means of support, fertility, and social reproduction among peasants: three approaches].

This article examines interpretations regarding the relationship between means of support and fertility, taking into account views on social reproduction among peasants, with emphasis on the way they are expressed in terms of indicators. The author begins with a brief summary of theories put forth by Malthus, and then examines Neo-Malthusian models which use as an independent variable, the amount of land and as a dependent variable, the number of children. He notes the existence of a positive ratio between these variables, and then demonstrates that in Chayanov's model, the number of children is used as an independent variable, while the idea of a positive ratio with the amount of land is still maintained. He compares these assumptions with those of Palerm, who suggests the existence of an inverse ratio between the amount of land available and fertility; this leads to policy implications contrary to those arising from neo-Malthusian models. The paper ends with a discussion of assumptions about the rationality, motivations, and guiding values the different approaches attribute to economic and demographic behavior among peasants.

Agriculture↗
PubMedDate not supplied

[Relations between population, development, and poverty, according to Mexican university professors].

"With data from a survey applied to random samples of university professors in seven Mexican cities during the late eighties, this article examines professors' opinions on the relations between population, development, and poverty. After reviewing tendencies and arguments on population and development found in [the] literature, it provides three simple typologies of population consciousness, poverty determinants, and best economic development plans for the country. It then studies their relations, and concludes by outlining the type of reasoning in each of the main orientations detected by the analysis." (SUMMARY IN ENG)

Americas↗