It is generally expected that, in the case of multiple herbicide resistance, different resistance mechanisms within a weed will follow Mendel’s law of independent assortment. Research was conducted to investigate anecdotal observations suggesting that target site–based resistances to inhibitors of acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO) did not follow independent assortment in common waterhemp. Cosegregation of the two resistances was observed in backcross lines (population sensitive to both herbicides as recurrent parent). Specifically, whereas 52% of backcross plants were resistant to a PPO inhibitor, this percentage increased to 92% when the backcross plants were preselected for resistance to an ALS inhibitor. Molecular marker analysis confirmed that the corresponding genes (ALS and PPX2) were genetically linked. When data from all plants analyzed were pooled, the genetic distance between the two genes was calculated to be 7.5 cM. The two genes were found to be about 195 kb apart in the recently published grain amaranth genome, explaining the observed genetic linkage. There is likely enough recombination that occurs between the linked genes to prevent the linkage from having significant implications in terms of resistance evolution. Nevertheless, documentation of the happenstance linkage between target-site genes for resistance to ALS and PPO inhibitors in waterhemp is a reminder that one should not assume distinct resistance mechanism will independently assort.

In this paper we present a method to generate independent samples for a general random variable, either continuous or discrete. The algorithm is an extension of the Acceptance-Rejection method, and it is particularly useful for kinetic simulation in which the rates are fluctuating in time and have singular limits, as occurs for example in simulation of recombination interactions in a plasma. Although it depends on some additional requirements, the new method is easy to implement and rejects less samples than the Acceptance-Rejection method.

The diversity in a 925 bp portion of the Plasmodium vivax MSA1 gene in isolates from the Philippines, China, the Solomon Islands and Papua New Guinea was investigated. A total of 74 base pair changes was found in the amplified fragment from 18 isolates. Most of these changes were single or double base pair substitutions. In several regions, these point changes were tightly linked with one set always present or always absent in the different isolates. Seven such blocks were identified. These blocks were present in different combinations in the different isolates indicating that extensive intragenic recombination has occurred.